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Simultaneous measurement of glomerular filtration rate and renal plasma flow using plasma disappearance curves
November, 1973 The ]ournal 6/ PEDIATRICS
749
Simultaneous measurement of glomerular filtration rate and renal plasma flo using plasma disappearance curves A clearance method based on the analysis of a disappearance curve of suitable substances/rom the plasma was u s e d / o r the purpose of determining the renal function of 99 children, ranging in age from 6 months to 17 years, with known or suspected renal disease. The disappearance of leSI-sodium iothalamate and 131I-orthoiodohippurate after a single injection was tested in 75 children, and was found to yield results "which correlated closely with the clearance obtained by continuous infusio~ of inulin ( Y ~ 0.93X + 14.5, r ~-- + 0.87) and p-aminohippuric acid ( Y -~- 0.80X + 38.6, r ~ + 0.93), respectively. The radiation dose for the combined procedure was estimated to be less than 0.133 millirad per microcurie to the whole body and 0.25 millirad per microcurie to the kidneys. The plasma disappearance curve following an injection of "cold" inulin was also found to provide an accurate estimate of the glomerular filtration rate, when compared with the clearance of endogenous creatinine ( Y ~- 1.07X + 0.3, r = + 0.89). The single injection method, with the important advantage of obviating the need for urine collection, appears to offer a reliable alternative to the standard clearance technique when appropriate regression equations are applied. The choice between the use of radioactive substances and inulin should be based on consideration of/actors such as the n e e d / o r repeat testing, availability of isotope-counting equipment, and accessibility of reliable chemical determinations.
G. I. Silkalns, M.D., D. Jeck, M.D., J. Earon, M.D., C. M. Edelmann, Jr., M.D., L. R. Chervu, Ph.D., M. D. Blaufox, M.D., and A. Spitzer, M.D., ~ B r o n x , N . Y .
From the Departments of Pediatrics, Medicine, and Radiology, and the Rose F. Kennedy Center for Research in Mental Retardation and H u m a n Development, The Albert Einstein College of Medicine. Supported in part by United States Public Health Service Grants No. A M 14,877, H.L. 05267, H D 01799, and H L 11,984; The Kidney Foundation of New York, Inc.; The Health Research Council of New York, 1-605; and The Sylvan League, Inc. ~Reprint address: Rose .F. Kennedy Center, 1410 Pelham Parkway South, Bronx, N. Y. 10461.
T H E M E A S U R E M E N T of glomerular filtration rate and effective renal plasma flow with standard clearance techniques is particularly difficult in patients of the pediatric age group. The main drawback of these methods is the need for complete, accurately timed urine collections and continuous intravenous infusions in subjects in whom good cooperation is frequently difficult to achieve. The task is virtually impossible in many paVol. 83, No. 5, pp. 749-757
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Silkalns et al.
tients with obstructive uropathy or therapeutic urinary diversion, encountered more and more frequently in pediatric practice. Compounding the problem is the tedious methodology required for the chemical determination of some of the substances used as markers of glomerular filtration (e.g., inulin) and the inaccuracy of the measurement of some of these substances at low concentration (e.g., creatinine). A method obviating the need for constant infusion and urine collections and, under certain circumstances, the need for chemical measurements would have definite advantages. Extensive studies have been performed in animals and in adult subjects in order to establish if glomerular filtration rate and effective renal plasma flow can be estimated accurately from the rate of disappearance from the blood of an appropriate substance following a single intravenous injection? -7 These studies have employed various radionuclides, and the data from the resultant plasma disappearance curves have been analyzed according to compartmental analysisY This method has been shown to yield results which correlate well with simultaneously performed standard clearances of inulin, p-aminohippuric acid (PAH), or endogenous creatinine. 9-12 Relatively little information is available, however, concerning the reliability of these methods in children? -4' 13, ~, Moreover, insufficient attention has been given to the problem of radiation, although in all of the studies cited radioactive materials were utilized. The present study was designed to determine in children the degree of accuracy of the single injection method when compared with classical clearance techniques, to obtain much needed information regarding the expected dose of radiation received by target organs, and to examine the use of the disappearance curve of "cold" inulin for estimating glomerular filtration rate. The results of the study affirm that the single injection method, based on radioactive or chemical measurements, accurately estimates glomerular filtration rate and effective renal plasma flow. Furthermore, the informa-
tion obtained allows the clinician to make an intelligent choice between the difficulty of the chemical determination of inulin and the hazards of radiation from various radionuclides. METHOD
Two protocols were followed. In the first, glomerular filtration rate and effective renal plasma flow were estimated separately or simultaneously from the plasma disappearance of l'-'sI-iothalamate (Glofil, Abbot Laboratories) and lalI-orthoiodohippurate (Hipputope, E. R. Squibb), respectively. The results obtained were compared to those of inulin, PAH, and endogenous creatinine clearances derived by standard methodology. The dosage of radionuclide employed was 20 microcuries of lzsI-iothalamate per 1.73 M. 2 and 40 microcuries of ~alI-orthoiodohippurate per 1.73 M?, with appropriate reduction in dosage, not exceeding 50 per cent in children with decreased renal function. In the second protocol, glomerular filtration rate estimated from the plasma disappearance of "cold" inulin after a single injection was compared to standard endogenous creatinine clearance. The dose of inulin was 0.5 ml. of a 10 per cent solution (50 mg.) per kilogram of body weight. The appropriate dose of radionuclide was drawn up into a 1 ml. disposable plastic syringe. An aliquot of approximately the same volume as the actual dose was injected into a volumetric flask, diluted to 500 ml., and used as a standard. The radioactivity in the total dose given to the patient (D) was calculated according to the formula: D=
(CPM/ml. aliquot) x (500) x (weight of dose) Weight of aliquot
where CPM = counts per minute. A similar method was used to determine the dose of inulin administered in the single injection studies. The exact amounts of inulin or radiopharmaceuticals used for the patient dose and for the standard were determined by weighing the corresponding syringe and needle before and after injection. The syringe
Volume 83 Number 5
Measuring glomerular filtration and renal plasma flow
was used only to inject and was not used to withdraw fluid from the patient. With this technique, the radioactivity in the needle did not affect the calculation of the dose. T h e radiopharmaceuticals or the inulin were injected rapidly into the intravenous infusion tubing, which was first clamped proximal to the injection site. The m o m e n t of injection, which was marked time zero, corresponded with the initiation of the first of three 20 minute urine collection periods for the inulin, PAH, or creatinine clearances. Eight heparinized, 2.5 ml. blood samples were collected at approximately 5, 10, 15, 20, 30, 40, 50, and 60 minutes after injection. I n the radionuclide studies, aliquots of 0.2 to 1.0 ml. of each plasma sample were counted in an A u t o - G a m m a Scintillation Counter (Nuclear Chicago). Each sample was counted to 10,000 counts, and C P M per jmilliliter were calculated. When two' isotopes were used simultaneously, correction was made for overlapping in the 12~I channel. No correction was necessary in the 1'~1I window because the 125I has a m u c h lower g a m m a energy. I n all patients receiving radioiodinated compounds, Lugol's solution was given prior to injection to block the thyroid uptake of any free radioactive iodine contained in the test substance. For the performance of standard clearances, inulin and P A H were administered by the constant infusion technique, employing Sigmamotor pumps. The infusion of the maintenance solution, which was calculated to maintain constant blood levels of inulin and P A H based on the glomerular filtration rate predicted from previous clearance measurements or serum creatinine concentration, was preceded by a loading dose of inulin and PAH. Urine collections were obtained by either voluntary voiding or bladder catheterization, depending upon the ability of the child to cooperate. Patients in whom bladder catheterization was performed were clinically in need of accurate assessment of renal function. Informed consent was obtained from all patients.
751
3000-
12sI-IOTHALAMATEDISAPPEARANCE
2000 -
( ml/min) IO
9
1000
5O0
E r~
\
C = Dose b I b z
100
~ 0
I
I
10
20
2: 0.173
I
I
30 40 MINUTES
I
1
50
60
Fig. 1. Plasma disappearance curve of 12aI-iothalamate with time. The curvillnear data (closed circles) are plotted from the actual plasma values. The terminal linear component is drawn by inspection, and then subtracted from the original data to determine the early linear component (open circles). The slopes and intercepts of these two components are used to calculate the clearance. Tile concentration of inulin in the blood and urine samples was determined by the method of Walser and associates 1~ and P A H by the method of Smith and associates, n~ Creatinine was determined on the Technieon AutoAnalyzer57 The single injection clearance was calculated on the basis of the model proposed by Sapirstein and associates. 8 T h e plasma disappearance curve was plotted on semilogarithmic graph paper as milligrams of inulin or C P M of isotope per milliliter of plasma on the ordinate (logarithm) against time in minutes on the abscissa (Fig. 1). T h e disappearance curve was then resolved into two linear components. T h e slope of the so-called slow component was obtained by drawing a tangent to the terminal portion of the original curve, which approaches a straight line. This slow component was then subtracted from the early portion of the curve to determine the more rapid early component.
752
SiIkalns et al.
The Journal o[ Pediatrics November 1973
220 200 E
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INULIN CLEARANCE (ml/min/1.73 rn2 )
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40 60 S0 100 120 140 160 CREATININE CLEARANCE(ml/min/1,73m2)
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Fig. 2. Regression of ~2~I-iothalamate disappearance on inulin clearance, both expressed as milliliter per minute per 1.73 M. 2. Regression equation: Y = 0.93X + 14.5, n = 61. The regression coefficient is significantly less than 1.0 (p = 0.01-0.02), but the intercept is not significantly different from zero (p > 0.2). Correlation coefficient r = + 0.89 (p < 0.001).
Fig. 3. Regression of 125I-iothalamate disappearance on creatinine clearance, both expressed as milliliter per minute per 1.73 M. 2. Regression equation: Y = 0.92X + 10.5, n = 56. The regression coefficient is significantly less than 1.0 (p -= 0.025), but the intercept is not significantly different from zero (p > 0.2). Correlation coefficient r = + 0.87 (p < 0.001).
T h e values resolved graphically were then used to calculate the clearance ( C ) :
nation of the regression, however, demonstrates t h a t this overestimate is only 5 p e r cent when g l o m e r u l a r filtration rate is 120, increases to 22 per cent when g l o m e r u l a r filtration rate is 50, a n d is 90 p e r cent when glomerular filtration rate is 15 ml. p e r minute per 1.73 M. ~. T h e regression of i o t h a l a m a t e on creatinine was quite similar to that of inulin a n d the correlation coefficient was the same. T h e m e a n clearance ratio was 1.04 _+ 0.028, reflecting the fact that the clearance of ereatinine also overestimates glomerular filtration rate (Fig. 3). Disappearance of l a l I - o r t h o i o d o h i p p u r a t e . Comparisons of the clearances calculated from the p l a s m a d i s a p p e a r a n c e curves with the s t a n d a r d clearance of P A H are shown in Fig. 4. T h e clearance ratio of 0.90 _+ 0.03 demonstrates that 131I-orthoiodohippurate underestimates CvAI~ by an average of 10 p e r cent. However, analysis of the e q u a t i o n describing the regression line indicates that 131I-orthoiodohippurate will underestimate the CpAH only above a value of a r o u n d 200 ml. per m i n u t e p e r 1.73 M. 2 a n d it will in-
C
=-
(D) x (bl b~), A b e + Bb~
where D is the total dose of inulin (in milligrams) or of the t r a c e r substance (in C P M ) given to the p a t i e n t ; A a n d bl are the intercept and the slope of the slow terminal component, a n d B a n d b2 are the intercept a n d the slope of the r a p i d early c o m p o n e n t of the p l a s m a d i s a p p e a r a n c e curve. Calculations of regressions, correlation coefficients, and significance of ratios were performed according to s t a n d a r d statistical m e t h o d s ? s Values are given as m e a n _+ 1 S.E. RESULTS
D i s a p p e a r a n c e of l e s I - i o t h a l a m a t e . T h e correlation between this m e t h o d and the clearance of inulin is high (r = + 0.87). T h e m e a n ratio of i o t h a l a m a t e clearance to inulin clearance was 1.12 _+ 0.035, i n d i c a t i n g t h a t on the average the single injection m e t h o d overestimates g l o m e r u l a r filtration rate by a p p r o x i m a t e l y 12 p e r cent (Fig. 2). E x a m i -
Volume 83 Number 5
Measuring glomerular filtration and renal plasma flow
e /
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El20 100
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Fig. 4. Regression of ~slI-orthoiodohippurate disappearance on PAH clearance, both expressed as milliliter per minute per 1.73 M. 2. Regression equation: Y ~ 0.80X + 38.6, n ~ 29. The regression coefficient is not significantly different from 1.0 (p ~- 0.2), ancj the intercept is not significantly different from zero (p > 0.20), indicating that the line of regression is not significantly different from the line of identity. Correlation coefficient r -----+ 0.93 (p ~ 0.001).
Fig. 5. Regression of inulin disappearance on creatinine clearance, both expressed as milliliter per minute per 1.73 MY. Regression equation: Y = 1.07X + 0.3, n = 24. The regression coefficient is not significantly different from 1.0 (p = 0.40), and the intercept is not significantly different from zero (p > 0.90), indicating that the line of regression is not significantly different from the line of identity. Correlation cdefficient, r = + 0.89 (p < 0.001).
creasingly overestimate it below this value. A t a CpA H of 600 ml. p e r m i n u t e p e r 1.73 M. z for instance, l s l I - o r t h o i o d o h i p p u r a t e will be 519 (86 p e r cent), whereas at a Ct, AH of 50, it will be 79 (157 p e r c e n t ) . D i s a p p e a r a n c e of inulin. C o m p a r i s o n of the clearance of inulin based on its p l a s m a d i s a p p e a r a n c e a n d that calculated from a conventionally~ p e r f o r m e d clearance of endogenous creatinine is shown on Fig. 5: T h e correlation coefficient is h i g h (r = +' 0.89), the m e a n of the clearance ratios is n o t significantly different from 1.0, a n d the inter= cept of the slope i s n o t different f r o m zero.
c o m p a r t m e n t a l analysis available at the time. T h e m o d e l proposed by Sapirstein a n d associ/ttesS: a n d the d e r i v a t i o n of a simple m a t h e matical f o r m u l a for the calculation of the renal clearance p a v e d the way for the numerous a n d mostly successful studies designed to validate this m e t h o d . This m o d e l is based on an open t w o - c o m p a r t m e n t system where the dose is injected into c o m p a r t m e n t one, diffuses freely between c o m p a r t m e n t one and c o m p a r t m e n t two, a n d is excreted only from c o m p a r t m e n t one (open m a m i l l a r y system). T h e basic assumptions inherent to this a p p r o a c h are as f o l l o w s : (1) the entire a m o u n t o f the m a t e r i a l used as a :marker is i n j e c t e d - i n k i a l l y into a single c o m p a r t m e n t , (2) the si,ze of the c o m p a r t m e n t s remain constant during the study, (3) m i x i n g is instantaneous, (4) the injected substance is free of c o n t a m i n a t i n g radioactivity, a n d . ( 5 ) the dose is excreted exclusively b y the kidney. T h e first two of t h e s e requirements are satis~ fled enti.relT: A l t h o u g h m i x i n g is not instan-taneous; the overestimate of the clearance which' results from this assumption probably
DISCUSSION V a l i d i t y of single injection clearances. A t tempts to use. the rate of d i s a p p e a r a n c e of substances injected into the bloodstream as indicators of the r a t e of g l o m e r u l a r filtration are not new. A long l i s t of references a n d a detailed discussion o f the theoretical background was ~ p r 6 v i d e d 25 years ago by Smith.lP The, m a i n reason why the early attempts 'failed was, t h e l i m i t e d ' k n o w l e d g e of
754
Silkalns et al.
does not exceed 3 per cent. 2~ The amount of radioactivity in commercially available materials usually is negligible. Finally, none of the substances used for measurements of renal function are cleared exclusively by the kidney. Hippuran is known to be excreted in bile2 Data on iothalamate and inulin are sparse? 2 However, increasing overestimation of the glomerular filtration rate as renal function falls suggests that removal of these substances occurs also by nonrenal routes. A potential problem encountered with the use of 12sI-iothalamate relates to its binding to plasma protein, which could cause falsely low clearances. Reports in the literature are at variance. Maher and Tauxe 21 reported that 8 to 27 per cent of iothalamate was plasma bound, but attempts to correct for this phenomenon resulted in clearance values which exceeded significantly those of inulin. Without this correction the clearance ratios were similar. The authors suggested that the degree of plasma binding balanced a postulated amount of tubular secretion. Other studies, however, have shown plasma protein binding of l~'~I-iothalamate to be insignificantY 2 Protein binding of a'~q-orthoiodohippurate has been reported by Blaufox and co-workers s, z, '~ to be minimal. Burbank and associates 2a found that clearance of 13~Iorthoiodohippurate amounts to 91 per cent of the clearance of PAH, which they interpreted to reflect a lower extraction ratio of Hippuran. This finding was confirmed by I~ountz and associatesY 4 The distribution kinetics of inulin was studied by Schachter and associates, 2s by Gaudino and associates, 2,~ and by Robson and associatesY 7 These investigators have shown that identity between plasma and interstitial fluid concentration obtains only at one point, sometime between 40 and 60 minutes after injection. Prior to this time the concentration gradient favors preferential movement of inulin toward the interstitial space, whereas following that point the converse applies. Although it appears that equilibrium between the two compartments is practically never reached, the impact of this phenomenon on the estimation of the renal clearance appears to be rather small.
The ]ournalo[ Pediatrics November 1973
Single injection clearances in children. This and other studies in children 1' 3, 4, 13 have shown that the single injection radionuclide clearance method can be performed satisfactorily in the pediatric patient, yielding results which correlate well with the generally accepted standard clearance techniques of inulin, PAH, and endogenous creatinine. The linear relationships which exist between these methods and those based On standard clearances allow the use of the regression equations for the calculation of glomerular filtration rate and renal plasma flow at any given level of renal function. The advantage of eliminating the collection of multiple urine specimens is obvious. In the present study, in which blood sampies were obtained for 60 minutes after injection, 12'~I-iothalamate was found to overestimate the clearance of inulin over the entire range of glomerular filtration rate studied. The explanation for this phenomenon has not been established. The virtual identity of inulin and iothalamate clearance by continuous infusion makes tubular secretion unlikely. 2s-~~ Other possibilities include biliary excretion of iothalamate and early loss in the urine due to incomplete mixing. A failure of equilibration and achievement of a true exponential relationship in the two volumes of distribution, as suggested by Cohen and associates, 4 must be considered. Preliminary data from our laboratory confirm that a higher degree of accuracy is reached when the study is carried out to 120 or 150 minutes. The improved precision of the longer study thus must be balanced against its inconvenience. Only one attempt was made in the past to estimate glomerular filtration rate in children from the disappearance from blood of a single dose of inulinY ~ The analysis was based on a one-compartment model. In a more recent study in which polyfructosan was substituted for inulin, the lack of concomitant clearance measurements with a dif~ .ferent method makes impossible the interpretation of the data? 2 T h e results we obtained indicate that the clearance calculated from disappearance of inulin from the plasm a approximates very closely the clearance
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Measuring gIomerular filtration and renal plasma flow
755
Table I. Absorbed dose (millirads/microcurie) from administered nuclide in patients with normal renal function
Age (yr.)
Body weight (Kg.)
1 5 10
10 20 35
1 5 l0
10 20 35
Absorbed dose* Bladderw Kidneys* a82 b#
Average dose (~c) 125I-iothalamate
Total bodyt
Gonadsl] a82 b#
7.2 8.7 11.6
0.02 0.01 0.006
0.03 0.02 0.015
0.13 0.08 0.07
0.26 0.15 0.10
0.01 0.01 0.02
0.09 0.10 0.09
0.11 0,05 0'.03
0.22 0.14 0.10
0.93 0.57 0.38
1.7 0.9 0.65
0.02 0.03 0.03
0.23 0.22 0.26
131I-orthoiodohippurate 10.4 17.4 23.2
e O r g a n weights and sizes were obtained f r o m Spector. 4~ T h e data pertaining to H / R values the equivalent radii and absorbed fractions were taken from standard sources. 41-4a T h e decay scheme data for the two radionuclides were obtained f r o m l)illman. 44 tBiologic half-life taken as equal to the terminal component; for 3 hours without voiding.
dose from bladder calculated on basis of accumulation of urine
:~The m e a n transit time through the kidneys was considered to be about 2 minutes. 45, 4~ w
of bladder taken to be 15, 20, and 30 Gm. at 1, 5, and 10 years.
IlOvaries considered to be located 4 cm. f r o m the center of t h e bladder. Calculations shown are for females; radiation dose 1o testes would he significantly lower. 82
based on emptying of bladder every 20 minutes.
# D o s e based on emptying of bladder every 90 minutes.
t of creatinine and can provide an estimate of glomerular filtration rate at least as accurate as that obtained with a25I-iothalamate. Although the use of inulin entails more complicated laboratory procedures, the method has the definite advantage of avoiding radiation exposure. Furthermore, the use of inulin permits the measurement of effective renal plasma flow, if desired, with 12aI-orthoiodohippurate, which provides only one-seventh the radiation dose of l'~lI-orthoiodohippurateY 3 An intelligent choice between these alternatives requires better knowledge of the amount of radiation received by the patient as a result of the procedures employing radionuclides. Radiation dosimetry. Calculation of radiation dosimetry for the procedures described in this article has been limited in most instances to "standard man"34; extension of these calculations to the pediatric age group has been very limited, a~-37 Cohen and associates la calculated that total body radiation after injection of 10 microcuries of 12GIiothalamate into a child weighing 30 Kg. is 0.25 millirad with normal glomerular filtration rate and 37 millirads with complete anuria. The radiation dose received by the target organs of children, although far more important than whole body radiation, has
not been estimated. As a consequence the problem is often approached from a philosophical rather than a scientific point of view. Not all the terms necessary to calculate the pediatric dosage are known, and a certain number of assumptions have to be made. Nevertheless, the results described here offer a reliable base for comparative judgment (Table I). The total dose to a 1-year-old child for the whole body and the ovaries of 1.2 and 0.25 millirad, respectively, can be compared to rates of external and internal radiation-from natural sources in normal regions of 2.53s and 2 millirads 39 per week, respectively. Seltzer and associates 33 have estimated the midline dose to the abdomen from an intravenous urogram in which five films are exposed to be 190, 210, and 480 millirads in children 1, 5, and 10 years old, respectively. The calculations indicate that with the amount of radioactive material employed in these studies and assuming accumulation in the bladder over a period of 3 hours, a 3 to 4 fold relative decrease in total body radiation occurs with the increase in age from 1 to 10 years. The difference is smaller with regard to the dose expected to be absorbed by kidneys, bladder, and gonads. It is important
756
Silkalns et al.
to note that the level of renal function does not influence greatly the radiation of the i n t r a - a b d o m i n a l organs, whereas the frequency of voiding has a significant i m p a c t on the a m o u n t of r a d i a t i o n absorbed by the gonads, which is assumed to be due m a i n l y to the g a m m a contribution of the activity a c c u m u l a t e d in t h e bladder. As a consequence, due to their greater distance from the bladder, the m a l e gonads receive considerably less dosage t h a n the female. Several conclusions can be d r a w n from the studies. (1) T h e single injection m e t h o d provides a simple a n d reliable alternative to the s t a n d a r d clearance technique for m e a surement of the g l o m e r u l a r filtration rate and a n effective renal p l a s m a flow. (2) T h e a m o u n t of r a d i a t i o n to the target organs, a n d in p a r t i c u l a r to the gonads, is low a n d can be kept so by a d e q u a t e l y h y d r a t i n g the patients a n d instructing t h e m to void frequently. (3) W h e r e v e r the chemical d e t e r m i n a t i o n of inulin can be p e r f o r m e d accurately, the disa p p e a r a n c e of this substance from the blood can be used successfully for the estimation of the g l o m e r u l a r filtration rate, obviating entirely the h a z a r d of radiation.
The ]ournal o[ Pediatrics November 1973
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8.
9.
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REFERENCES
1. Sakai, T., Leumann, E. P., and Holliday, M. A.: Single injection clearance in children, Pediatrics 44: 905, 1969. 2. Donath, A.: The simultaneous determination in children of glomerular filtration rate and effective renal plasma flow by the single injection clearance technique, Acta Paediatr. Scan& 60: 512, 1971. 3, VSgeli, B., Riedwyl, IL, Donath, A., and Oetliker, O.: Comparison of glomerular filtration rate and effective renal plasma flow determinations obtained by a single injection technique and by means of a standard clearance technique in children, Acts Paediatr. Scand. 60: 528, 197I. 4. Cohen, M. L., Patel, J. K., and Baxter, D. L.: External monitoring and plasma disappearance for the determination of renal function: Comparison of effective renal plasma flow and glomerular filtration rate, Pediatrics 48: 377, 1971. 5. Blaufox, M. D., Potchen, E. J., and Merrill, J. P.: Measurement of effective renal plasma flow in man by external counting methods, J. Nucl. Med. 8: 77, 1967. 6. Bianchi, C., Coli, A., and Palla, R.: 1~1I hypaque and external counting in glomeru-
15. 16.
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18. 19. 20.
21.
22.
lar filtration rate (GFR) measurement, Clin. Res. 15: 351, 1967. Blaufox, M. D., Guttmann, R. D., and Merrill, J. P.: Measurement of renal function in the rat with single injection clearances, Am. J. Physiol. 212: 629, 1967. Sapirstein, L. A., Vidt, D. G., Mandel, M. J., and Hanusek, G.: Volumes of distribution and clearances of intravenously injected creatinine in the dog, Am. J. Physiol. 181: 330, 1955. Blaufox, M. D., Orvis, A. L., and Owen, C. A., Jr.: Compartment analysis of the radiorenogram and distribution of Hippuran-1131 in dogs, Am. J. Physiol. 204: 1059, 1963. Farmer, C. D., Tauxe, W. N., Maher, F. T., and Hunt, J. C.: Measurement of renal function with radioiodinated diatrizoate and o-iodohippurate, Am. J. Clin. Pathol. 47: 9, 1967. Maher, F. T., and Elveback, L. R.: Simultaneous renal clearances of 12aI- and ls~Ilabelled orthoiodohippurate and para-aminohippurate in the estimation of effective renal plasma flow in man, Mayo Clin. Proc. 45: 657, 1970. Maher, F. T., Nolan, N. G., and Elveback, L. R.: Comparison of simultaneous clearances of ~25-I-labelled sodium iothalamate (Glofil) and of inulin, Mayo Clin. Proc. 46: 690, 1971. Cohen, M. L., Smith, F. G., Jr., Mindetl, R. S., and Vernier, R. L.: A simple, reliable method of measuring glomerular filtration rate using single, low dose sodium iothalamate I TM, Pediatrics 43: 407, 1969. Bianchi, C., Coli, A., Gallucci, L., Paci, A., Palla, R., and Rindi, P.: The measurement of glomerular filtration rate in children b y z31I hypaque and external counting, J. Nucl. Biol. Med. 11: 144, 1967. Walser, M., Davidson, D. G., and Orloff, J.: The renal clearance of alkali-stable inulin, J. Clin. Invest. 34: 1520, 1955. Smith, H. W., Finkelstein, N., Aliminosa, L., Crawford, B., and Graber, M.: The renal clearances of substituted hippuric acid derivatives and other aromatic acids in dogs and man, J. Clin. Invest. 24: 388, 1945. Bauer, J. D., Toro, G., and Ackermann, P. G., editors: Bray's clinical laboratory methods, ed. 6, St. Louis, 1962, The C. V. Mosby Company, p. 260. Snedecor, G. W.: Statistical methods applied to experiments in agricuIture and biology, Ames, 1956, Iowa State University Press. Smith, H. W.: The kidney, New York, 1951, Oxford University Press, p. 39. Sigman, E. M., Elwood, C. M., and Knox, F.: The measurement of glomerular filtration rate in man with sodium iothalamate lalI (Conray), J. Nucl. Med. 7: 60, 1966. Maher, F. T., and Tauxe, W. N.: Renal clearance in man of pharmaceuticals containing radioactive iodine: Influence of plasma binding, J. A. M. A. 207- 97, 1969. Anderson, C. F., Sawyer, T. K., and Cutler,
Volume 83 Number 5
Measuring glomerular filtration and renal plasma flow
R. E.: Iothalamate sodium I a25 vs. cyanocobalamin Co ~z as a measure of glomerular filtration rate in m a n , J. A. M. A. 204: 105, 1968. 23..Burbank, M. K., Tauxe, W. N., Maher, F. T., and Hunt, J. C.: Evaluation of radioiodinated hippuran for the estimation of renal plasma flow, Mayo Clin. Proc. 36: 372, 1961. 24. Kountz, S. L., Truex, G., Earley, L. E., and Belzer, F. O.: Serial hemodynamics after renal allotransplantation in man, Circulation 41: 217, 1970. 25. Schaehter, D., Freinkel, N., and Schwartz, I. L.: Movement of inulin between plasma and interstitial fluid, Am. J. Physiol. 160: 532, 1950. 26. Gaudino, M., Schwartz, I. L,, and Levitt, M. F.: Inulin volume of distribution as a measure of extracellular fluid in dog and man, Proc. Soc. Exp. Biol. Med. 68: 507, 1948. 27. Robson, J. S., Ferguson, M. H., Olbrich, O., and Steward, L. P.: The determination of the renal clearance of inulin in man Q. J. Exp. Physiol. 35: 111, 1949. 28. Sigman, E. M., Elwood, C., Reagan, M. E., Morris, A. M., and Catanzaro, A.: The renal clearance of IlSMabelled sodium iothalamate in man, Invest! Urol. 2: 432, 1965. 29. Griep, . R . J . , knd Nelp, W. B.: Mechanism of renal excretion of iodine-1~1 sodium iothalamate (Conray), J. Nucl. Med. 7: 370, 1966. 30. Elwood, C. M., Sigman, E. M., and Treger, C.: The measurement of glomerular filtration rate with ~2~-I-sodium iothalamate (Conray), Br. J. Radlol. 40: 581, 1967. 31. Barnett, H. L.: Renal physiology in infants and children. I. Method for estimation of glomerular filtration rate, Proc. Soc. Exp. Biol. Med. 44: 654, 1940. 32. Geisert, J., Willard, D., Saerez, R., Sigrist, D., and Mack, G.: Clearance glomerulaire et fraction de filtration ehez l'enfant, Arch. Fr. Pediatr. 26: 125, 1969. 33. Seltzer, R. A., Kereiakes, J. G., and Senger, E. L.: Radiation exposure from radio-isotopes in pediatrics, N. Engl. J. Med. 271: 84, 1964. 34. Morgan, K. Z.: Standard man--standard patient, in Cloutier, R. J., Edwards, C. L., and Snyder, W. S., editors : Medical radionuclides : Radiation dose and effects, Proceedings of Symposium, United States Atomic Energy Commission, 1970.
757
35. Kerelakes, J. G., Wellman, H. N., Tieman, J., and Saenger, E. L. A.: Radiopharmaceutical dosimetry in pediatrics, Radiology 90: 925, 1968. 36. Wellman, H. N., Kereiakes, J. G., and Branson, B. M.: Total and partial body counting of children for radiopharmaeeutical dosimetry data, in Cloutier, R. J., Edwards, C. L., and Snyder, W. S,, editors: Medical radionuclides: Radiation dose and effects, Proceedings of Symposium, United States Atomic Energy Commission, 1970. 37. Tefft, M.: More common radionuclide examination in children: Indications for use with a discussion of radiation dose received, Pediatrics 48: 802, 1971. 38. Harley, J. H.: in S,ax, N. Irving, editor: Dangerous properties of industrial materials, New York, 1968, Reinhold Book Corp. 39. Henry, H. F.: Fundamentals of radiation protection, New York, 1969, Interscience Publishers, Inc., p. 101. 40. Speetor, W. S.: Handbook of biological data, body and organ weights: Man, Philadelphia, 1956, W. B. Saunders Company, p. 162, Table 132. 41. Powsner, E. R., and Raeside, D. E.: Diagnostic nuclear medicine, New York, 1971, Grune & Stratton, Inc. 42. Snyder, W. S., Ford, M. R., Warner, G. C., and Fisher, H. L., Jr.: Estimates of absorbed fractions for monoenergetic photon sources uniformly distributed in various organs of a heterogeneous phantom, J. Nucl. Med. 10: SuppI. 3, 5, 1969. 43. Ellet, W. H., and Humes, R. M.: Absorbed fractions for small volumes containing photon emitting radioactivity, J'. Nucl. Med. 12: Suppl. 5, 25, 1971. 44. Dillman, L. T.: Radionuclide decay schemes and nuclear parameters for use in radiation dose estimation, J. Nucl. Med. 10: Suppl. 2, 5, 1969. 45. McSwiney, R. R., and de Wardner, H. E,: Renal-tract delay time and dead space, Lancet 2: 845, 1950. 46. Blaufox, M. D., and Conroy, M. F.: Measurement of renal mean transit time of Hippuran 131I with external counting, J. Nucl. Biol. Med. 12: 107, 1968.
749
Simultaneous measurement of glomerular filtration rate and renal plasma flo using plasma disappearance curves A clearance method based on the analysis of a disappearance curve of suitable substances/rom the plasma was u s e d / o r the purpose of determining the renal function of 99 children, ranging in age from 6 months to 17 years, with known or suspected renal disease. The disappearance of leSI-sodium iothalamate and 131I-orthoiodohippurate after a single injection was tested in 75 children, and was found to yield results "which correlated closely with the clearance obtained by continuous infusio~ of inulin ( Y ~ 0.93X + 14.5, r ~-- + 0.87) and p-aminohippuric acid ( Y -~- 0.80X + 38.6, r ~ + 0.93), respectively. The radiation dose for the combined procedure was estimated to be less than 0.133 millirad per microcurie to the whole body and 0.25 millirad per microcurie to the kidneys. The plasma disappearance curve following an injection of "cold" inulin was also found to provide an accurate estimate of the glomerular filtration rate, when compared with the clearance of endogenous creatinine ( Y ~- 1.07X + 0.3, r = + 0.89). The single injection method, with the important advantage of obviating the need for urine collection, appears to offer a reliable alternative to the standard clearance technique when appropriate regression equations are applied. The choice between the use of radioactive substances and inulin should be based on consideration of/actors such as the n e e d / o r repeat testing, availability of isotope-counting equipment, and accessibility of reliable chemical determinations.
G. I. Silkalns, M.D., D. Jeck, M.D., J. Earon, M.D., C. M. Edelmann, Jr., M.D., L. R. Chervu, Ph.D., M. D. Blaufox, M.D., and A. Spitzer, M.D., ~ B r o n x , N . Y .
From the Departments of Pediatrics, Medicine, and Radiology, and the Rose F. Kennedy Center for Research in Mental Retardation and H u m a n Development, The Albert Einstein College of Medicine. Supported in part by United States Public Health Service Grants No. A M 14,877, H.L. 05267, H D 01799, and H L 11,984; The Kidney Foundation of New York, Inc.; The Health Research Council of New York, 1-605; and The Sylvan League, Inc. ~Reprint address: Rose .F. Kennedy Center, 1410 Pelham Parkway South, Bronx, N. Y. 10461.
T H E M E A S U R E M E N T of glomerular filtration rate and effective renal plasma flow with standard clearance techniques is particularly difficult in patients of the pediatric age group. The main drawback of these methods is the need for complete, accurately timed urine collections and continuous intravenous infusions in subjects in whom good cooperation is frequently difficult to achieve. The task is virtually impossible in many paVol. 83, No. 5, pp. 749-757
750
The Journal o[ Pediatrics November 1973
Silkalns et al.
tients with obstructive uropathy or therapeutic urinary diversion, encountered more and more frequently in pediatric practice. Compounding the problem is the tedious methodology required for the chemical determination of some of the substances used as markers of glomerular filtration (e.g., inulin) and the inaccuracy of the measurement of some of these substances at low concentration (e.g., creatinine). A method obviating the need for constant infusion and urine collections and, under certain circumstances, the need for chemical measurements would have definite advantages. Extensive studies have been performed in animals and in adult subjects in order to establish if glomerular filtration rate and effective renal plasma flow can be estimated accurately from the rate of disappearance from the blood of an appropriate substance following a single intravenous injection? -7 These studies have employed various radionuclides, and the data from the resultant plasma disappearance curves have been analyzed according to compartmental analysisY This method has been shown to yield results which correlate well with simultaneously performed standard clearances of inulin, p-aminohippuric acid (PAH), or endogenous creatinine. 9-12 Relatively little information is available, however, concerning the reliability of these methods in children? -4' 13, ~, Moreover, insufficient attention has been given to the problem of radiation, although in all of the studies cited radioactive materials were utilized. The present study was designed to determine in children the degree of accuracy of the single injection method when compared with classical clearance techniques, to obtain much needed information regarding the expected dose of radiation received by target organs, and to examine the use of the disappearance curve of "cold" inulin for estimating glomerular filtration rate. The results of the study affirm that the single injection method, based on radioactive or chemical measurements, accurately estimates glomerular filtration rate and effective renal plasma flow. Furthermore, the informa-
tion obtained allows the clinician to make an intelligent choice between the difficulty of the chemical determination of inulin and the hazards of radiation from various radionuclides. METHOD
Two protocols were followed. In the first, glomerular filtration rate and effective renal plasma flow were estimated separately or simultaneously from the plasma disappearance of l'-'sI-iothalamate (Glofil, Abbot Laboratories) and lalI-orthoiodohippurate (Hipputope, E. R. Squibb), respectively. The results obtained were compared to those of inulin, PAH, and endogenous creatinine clearances derived by standard methodology. The dosage of radionuclide employed was 20 microcuries of lzsI-iothalamate per 1.73 M. 2 and 40 microcuries of ~alI-orthoiodohippurate per 1.73 M?, with appropriate reduction in dosage, not exceeding 50 per cent in children with decreased renal function. In the second protocol, glomerular filtration rate estimated from the plasma disappearance of "cold" inulin after a single injection was compared to standard endogenous creatinine clearance. The dose of inulin was 0.5 ml. of a 10 per cent solution (50 mg.) per kilogram of body weight. The appropriate dose of radionuclide was drawn up into a 1 ml. disposable plastic syringe. An aliquot of approximately the same volume as the actual dose was injected into a volumetric flask, diluted to 500 ml., and used as a standard. The radioactivity in the total dose given to the patient (D) was calculated according to the formula: D=
(CPM/ml. aliquot) x (500) x (weight of dose) Weight of aliquot
where CPM = counts per minute. A similar method was used to determine the dose of inulin administered in the single injection studies. The exact amounts of inulin or radiopharmaceuticals used for the patient dose and for the standard were determined by weighing the corresponding syringe and needle before and after injection. The syringe
Volume 83 Number 5
Measuring glomerular filtration and renal plasma flow
was used only to inject and was not used to withdraw fluid from the patient. With this technique, the radioactivity in the needle did not affect the calculation of the dose. T h e radiopharmaceuticals or the inulin were injected rapidly into the intravenous infusion tubing, which was first clamped proximal to the injection site. The m o m e n t of injection, which was marked time zero, corresponded with the initiation of the first of three 20 minute urine collection periods for the inulin, PAH, or creatinine clearances. Eight heparinized, 2.5 ml. blood samples were collected at approximately 5, 10, 15, 20, 30, 40, 50, and 60 minutes after injection. I n the radionuclide studies, aliquots of 0.2 to 1.0 ml. of each plasma sample were counted in an A u t o - G a m m a Scintillation Counter (Nuclear Chicago). Each sample was counted to 10,000 counts, and C P M per jmilliliter were calculated. When two' isotopes were used simultaneously, correction was made for overlapping in the 12~I channel. No correction was necessary in the 1'~1I window because the 125I has a m u c h lower g a m m a energy. I n all patients receiving radioiodinated compounds, Lugol's solution was given prior to injection to block the thyroid uptake of any free radioactive iodine contained in the test substance. For the performance of standard clearances, inulin and P A H were administered by the constant infusion technique, employing Sigmamotor pumps. The infusion of the maintenance solution, which was calculated to maintain constant blood levels of inulin and P A H based on the glomerular filtration rate predicted from previous clearance measurements or serum creatinine concentration, was preceded by a loading dose of inulin and PAH. Urine collections were obtained by either voluntary voiding or bladder catheterization, depending upon the ability of the child to cooperate. Patients in whom bladder catheterization was performed were clinically in need of accurate assessment of renal function. Informed consent was obtained from all patients.
751
3000-
12sI-IOTHALAMATEDISAPPEARANCE
2000 -
( ml/min) IO
9
1000
5O0
E r~
\
C = Dose b I b z
100
~ 0
I
I
10
20
2: 0.173
I
I
30 40 MINUTES
I
1
50
60
Fig. 1. Plasma disappearance curve of 12aI-iothalamate with time. The curvillnear data (closed circles) are plotted from the actual plasma values. The terminal linear component is drawn by inspection, and then subtracted from the original data to determine the early linear component (open circles). The slopes and intercepts of these two components are used to calculate the clearance. Tile concentration of inulin in the blood and urine samples was determined by the method of Walser and associates 1~ and P A H by the method of Smith and associates, n~ Creatinine was determined on the Technieon AutoAnalyzer57 The single injection clearance was calculated on the basis of the model proposed by Sapirstein and associates. 8 T h e plasma disappearance curve was plotted on semilogarithmic graph paper as milligrams of inulin or C P M of isotope per milliliter of plasma on the ordinate (logarithm) against time in minutes on the abscissa (Fig. 1). T h e disappearance curve was then resolved into two linear components. T h e slope of the so-called slow component was obtained by drawing a tangent to the terminal portion of the original curve, which approaches a straight line. This slow component was then subtracted from the early portion of the curve to determine the more rapid early component.
752
SiIkalns et al.
The Journal o[ Pediatrics November 1973
220 200 E
200 e~ E
180 o~
._a 160
9
180 e~
9
i
9
9
9
9
140
9
9
Z
ta_
120
<
9
II Iii
9
120
o e~ ~
~00
100,
:E
IO g
Lu
.~
so
9
so 9
<' 60
I
b
-~ 40 i
z
60
40
c~
20 0
8
160 E
140 <
.../
20 I 20
I 40
I 60
I S0
I 100
I 120
I 140
I 160
[ ~80
I 200
INULIN CLEARANCE (ml/min/1.73 rn2 )
I
0
20
I
I
I
I
I
I
I
40 60 S0 100 120 140 160 CREATININE CLEARANCE(ml/min/1,73m2)
I
180
I
200
Fig. 2. Regression of ~2~I-iothalamate disappearance on inulin clearance, both expressed as milliliter per minute per 1.73 M. 2. Regression equation: Y = 0.93X + 14.5, n = 61. The regression coefficient is significantly less than 1.0 (p = 0.01-0.02), but the intercept is not significantly different from zero (p > 0.2). Correlation coefficient r = + 0.89 (p < 0.001).
Fig. 3. Regression of 125I-iothalamate disappearance on creatinine clearance, both expressed as milliliter per minute per 1.73 M. 2. Regression equation: Y = 0.92X + 10.5, n = 56. The regression coefficient is significantly less than 1.0 (p -= 0.025), but the intercept is not significantly different from zero (p > 0.2). Correlation coefficient r = + 0.87 (p < 0.001).
T h e values resolved graphically were then used to calculate the clearance ( C ) :
nation of the regression, however, demonstrates t h a t this overestimate is only 5 p e r cent when g l o m e r u l a r filtration rate is 120, increases to 22 per cent when g l o m e r u l a r filtration rate is 50, a n d is 90 p e r cent when glomerular filtration rate is 15 ml. p e r minute per 1.73 M. ~. T h e regression of i o t h a l a m a t e on creatinine was quite similar to that of inulin a n d the correlation coefficient was the same. T h e m e a n clearance ratio was 1.04 _+ 0.028, reflecting the fact that the clearance of ereatinine also overestimates glomerular filtration rate (Fig. 3). Disappearance of l a l I - o r t h o i o d o h i p p u r a t e . Comparisons of the clearances calculated from the p l a s m a d i s a p p e a r a n c e curves with the s t a n d a r d clearance of P A H are shown in Fig. 4. T h e clearance ratio of 0.90 _+ 0.03 demonstrates that 131I-orthoiodohippurate underestimates CvAI~ by an average of 10 p e r cent. However, analysis of the e q u a t i o n describing the regression line indicates that 131I-orthoiodohippurate will underestimate the CpAH only above a value of a r o u n d 200 ml. per m i n u t e p e r 1.73 M. 2 a n d it will in-
C
=-
(D) x (bl b~), A b e + Bb~
where D is the total dose of inulin (in milligrams) or of the t r a c e r substance (in C P M ) given to the p a t i e n t ; A a n d bl are the intercept and the slope of the slow terminal component, a n d B a n d b2 are the intercept a n d the slope of the r a p i d early c o m p o n e n t of the p l a s m a d i s a p p e a r a n c e curve. Calculations of regressions, correlation coefficients, and significance of ratios were performed according to s t a n d a r d statistical m e t h o d s ? s Values are given as m e a n _+ 1 S.E. RESULTS
D i s a p p e a r a n c e of l e s I - i o t h a l a m a t e . T h e correlation between this m e t h o d and the clearance of inulin is high (r = + 0.87). T h e m e a n ratio of i o t h a l a m a t e clearance to inulin clearance was 1.12 _+ 0.035, i n d i c a t i n g t h a t on the average the single injection m e t h o d overestimates g l o m e r u l a r filtration rate by a p p r o x i m a t e l y 12 p e r cent (Fig. 2). E x a m i -
Volume 83 Number 5
Measuring glomerular filtration and renal plasma flow
e /
200
8oo
753
180
9 9 o/
160
~ 600
El20 100
40C
I
g O I
200
f-
99
0/00 9
,4..
@
I
/
I
I
200 400 600 PAH CLEARANCE(ml/min/1.73m2)
_J
800
41 0~~21 /0 i * I ", I I I I I 0 20 40 60 80 100 120 140 160 180 200 CREATININE CLEARANCE (m[/min/1.73m2)
Fig. 4. Regression of ~slI-orthoiodohippurate disappearance on PAH clearance, both expressed as milliliter per minute per 1.73 M. 2. Regression equation: Y ~ 0.80X + 38.6, n ~ 29. The regression coefficient is not significantly different from 1.0 (p ~- 0.2), ancj the intercept is not significantly different from zero (p > 0.20), indicating that the line of regression is not significantly different from the line of identity. Correlation coefficient r -----+ 0.93 (p ~ 0.001).
Fig. 5. Regression of inulin disappearance on creatinine clearance, both expressed as milliliter per minute per 1.73 MY. Regression equation: Y = 1.07X + 0.3, n = 24. The regression coefficient is not significantly different from 1.0 (p = 0.40), and the intercept is not significantly different from zero (p > 0.90), indicating that the line of regression is not significantly different from the line of identity. Correlation cdefficient, r = + 0.89 (p < 0.001).
creasingly overestimate it below this value. A t a CpA H of 600 ml. p e r m i n u t e p e r 1.73 M. z for instance, l s l I - o r t h o i o d o h i p p u r a t e will be 519 (86 p e r cent), whereas at a Ct, AH of 50, it will be 79 (157 p e r c e n t ) . D i s a p p e a r a n c e of inulin. C o m p a r i s o n of the clearance of inulin based on its p l a s m a d i s a p p e a r a n c e a n d that calculated from a conventionally~ p e r f o r m e d clearance of endogenous creatinine is shown on Fig. 5: T h e correlation coefficient is h i g h (r = +' 0.89), the m e a n of the clearance ratios is n o t significantly different from 1.0, a n d the inter= cept of the slope i s n o t different f r o m zero.
c o m p a r t m e n t a l analysis available at the time. T h e m o d e l proposed by Sapirstein a n d associ/ttesS: a n d the d e r i v a t i o n of a simple m a t h e matical f o r m u l a for the calculation of the renal clearance p a v e d the way for the numerous a n d mostly successful studies designed to validate this m e t h o d . This m o d e l is based on an open t w o - c o m p a r t m e n t system where the dose is injected into c o m p a r t m e n t one, diffuses freely between c o m p a r t m e n t one and c o m p a r t m e n t two, a n d is excreted only from c o m p a r t m e n t one (open m a m i l l a r y system). T h e basic assumptions inherent to this a p p r o a c h are as f o l l o w s : (1) the entire a m o u n t o f the m a t e r i a l used as a :marker is i n j e c t e d - i n k i a l l y into a single c o m p a r t m e n t , (2) the si,ze of the c o m p a r t m e n t s remain constant during the study, (3) m i x i n g is instantaneous, (4) the injected substance is free of c o n t a m i n a t i n g radioactivity, a n d . ( 5 ) the dose is excreted exclusively b y the kidney. T h e first two of t h e s e requirements are satis~ fled enti.relT: A l t h o u g h m i x i n g is not instan-taneous; the overestimate of the clearance which' results from this assumption probably
DISCUSSION V a l i d i t y of single injection clearances. A t tempts to use. the rate of d i s a p p e a r a n c e of substances injected into the bloodstream as indicators of the r a t e of g l o m e r u l a r filtration are not new. A long l i s t of references a n d a detailed discussion o f the theoretical background was ~ p r 6 v i d e d 25 years ago by Smith.lP The, m a i n reason why the early attempts 'failed was, t h e l i m i t e d ' k n o w l e d g e of
754
Silkalns et al.
does not exceed 3 per cent. 2~ The amount of radioactivity in commercially available materials usually is negligible. Finally, none of the substances used for measurements of renal function are cleared exclusively by the kidney. Hippuran is known to be excreted in bile2 Data on iothalamate and inulin are sparse? 2 However, increasing overestimation of the glomerular filtration rate as renal function falls suggests that removal of these substances occurs also by nonrenal routes. A potential problem encountered with the use of 12sI-iothalamate relates to its binding to plasma protein, which could cause falsely low clearances. Reports in the literature are at variance. Maher and Tauxe 21 reported that 8 to 27 per cent of iothalamate was plasma bound, but attempts to correct for this phenomenon resulted in clearance values which exceeded significantly those of inulin. Without this correction the clearance ratios were similar. The authors suggested that the degree of plasma binding balanced a postulated amount of tubular secretion. Other studies, however, have shown plasma protein binding of l~'~I-iothalamate to be insignificantY 2 Protein binding of a'~q-orthoiodohippurate has been reported by Blaufox and co-workers s, z, '~ to be minimal. Burbank and associates 2a found that clearance of 13~Iorthoiodohippurate amounts to 91 per cent of the clearance of PAH, which they interpreted to reflect a lower extraction ratio of Hippuran. This finding was confirmed by I~ountz and associatesY 4 The distribution kinetics of inulin was studied by Schachter and associates, 2s by Gaudino and associates, 2,~ and by Robson and associatesY 7 These investigators have shown that identity between plasma and interstitial fluid concentration obtains only at one point, sometime between 40 and 60 minutes after injection. Prior to this time the concentration gradient favors preferential movement of inulin toward the interstitial space, whereas following that point the converse applies. Although it appears that equilibrium between the two compartments is practically never reached, the impact of this phenomenon on the estimation of the renal clearance appears to be rather small.
The ]ournalo[ Pediatrics November 1973
Single injection clearances in children. This and other studies in children 1' 3, 4, 13 have shown that the single injection radionuclide clearance method can be performed satisfactorily in the pediatric patient, yielding results which correlate well with the generally accepted standard clearance techniques of inulin, PAH, and endogenous creatinine. The linear relationships which exist between these methods and those based On standard clearances allow the use of the regression equations for the calculation of glomerular filtration rate and renal plasma flow at any given level of renal function. The advantage of eliminating the collection of multiple urine specimens is obvious. In the present study, in which blood sampies were obtained for 60 minutes after injection, 12'~I-iothalamate was found to overestimate the clearance of inulin over the entire range of glomerular filtration rate studied. The explanation for this phenomenon has not been established. The virtual identity of inulin and iothalamate clearance by continuous infusion makes tubular secretion unlikely. 2s-~~ Other possibilities include biliary excretion of iothalamate and early loss in the urine due to incomplete mixing. A failure of equilibration and achievement of a true exponential relationship in the two volumes of distribution, as suggested by Cohen and associates, 4 must be considered. Preliminary data from our laboratory confirm that a higher degree of accuracy is reached when the study is carried out to 120 or 150 minutes. The improved precision of the longer study thus must be balanced against its inconvenience. Only one attempt was made in the past to estimate glomerular filtration rate in children from the disappearance from blood of a single dose of inulinY ~ The analysis was based on a one-compartment model. In a more recent study in which polyfructosan was substituted for inulin, the lack of concomitant clearance measurements with a dif~ .ferent method makes impossible the interpretation of the data? 2 T h e results we obtained indicate that the clearance calculated from disappearance of inulin from the plasm a approximates very closely the clearance
Volume 83 Number 5
Measuring gIomerular filtration and renal plasma flow
755
Table I. Absorbed dose (millirads/microcurie) from administered nuclide in patients with normal renal function
Age (yr.)
Body weight (Kg.)
1 5 10
10 20 35
1 5 l0
10 20 35
Absorbed dose* Bladderw Kidneys* a82 b#
Average dose (~c) 125I-iothalamate
Total bodyt
Gonadsl] a82 b#
7.2 8.7 11.6
0.02 0.01 0.006
0.03 0.02 0.015
0.13 0.08 0.07
0.26 0.15 0.10
0.01 0.01 0.02
0.09 0.10 0.09
0.11 0,05 0'.03
0.22 0.14 0.10
0.93 0.57 0.38
1.7 0.9 0.65
0.02 0.03 0.03
0.23 0.22 0.26
131I-orthoiodohippurate 10.4 17.4 23.2
e O r g a n weights and sizes were obtained f r o m Spector. 4~ T h e data pertaining to H / R values the equivalent radii and absorbed fractions were taken from standard sources. 41-4a T h e decay scheme data for the two radionuclides were obtained f r o m l)illman. 44 tBiologic half-life taken as equal to the terminal component; for 3 hours without voiding.
dose from bladder calculated on basis of accumulation of urine
:~The m e a n transit time through the kidneys was considered to be about 2 minutes. 45, 4~ w
of bladder taken to be 15, 20, and 30 Gm. at 1, 5, and 10 years.
IlOvaries considered to be located 4 cm. f r o m the center of t h e bladder. Calculations shown are for females; radiation dose 1o testes would he significantly lower. 82
based on emptying of bladder every 20 minutes.
# D o s e based on emptying of bladder every 90 minutes.
t of creatinine and can provide an estimate of glomerular filtration rate at least as accurate as that obtained with a25I-iothalamate. Although the use of inulin entails more complicated laboratory procedures, the method has the definite advantage of avoiding radiation exposure. Furthermore, the use of inulin permits the measurement of effective renal plasma flow, if desired, with 12aI-orthoiodohippurate, which provides only one-seventh the radiation dose of l'~lI-orthoiodohippurateY 3 An intelligent choice between these alternatives requires better knowledge of the amount of radiation received by the patient as a result of the procedures employing radionuclides. Radiation dosimetry. Calculation of radiation dosimetry for the procedures described in this article has been limited in most instances to "standard man"34; extension of these calculations to the pediatric age group has been very limited, a~-37 Cohen and associates la calculated that total body radiation after injection of 10 microcuries of 12GIiothalamate into a child weighing 30 Kg. is 0.25 millirad with normal glomerular filtration rate and 37 millirads with complete anuria. The radiation dose received by the target organs of children, although far more important than whole body radiation, has
not been estimated. As a consequence the problem is often approached from a philosophical rather than a scientific point of view. Not all the terms necessary to calculate the pediatric dosage are known, and a certain number of assumptions have to be made. Nevertheless, the results described here offer a reliable base for comparative judgment (Table I). The total dose to a 1-year-old child for the whole body and the ovaries of 1.2 and 0.25 millirad, respectively, can be compared to rates of external and internal radiation-from natural sources in normal regions of 2.53s and 2 millirads 39 per week, respectively. Seltzer and associates 33 have estimated the midline dose to the abdomen from an intravenous urogram in which five films are exposed to be 190, 210, and 480 millirads in children 1, 5, and 10 years old, respectively. The calculations indicate that with the amount of radioactive material employed in these studies and assuming accumulation in the bladder over a period of 3 hours, a 3 to 4 fold relative decrease in total body radiation occurs with the increase in age from 1 to 10 years. The difference is smaller with regard to the dose expected to be absorbed by kidneys, bladder, and gonads. It is important
756
Silkalns et al.
to note that the level of renal function does not influence greatly the radiation of the i n t r a - a b d o m i n a l organs, whereas the frequency of voiding has a significant i m p a c t on the a m o u n t of r a d i a t i o n absorbed by the gonads, which is assumed to be due m a i n l y to the g a m m a contribution of the activity a c c u m u l a t e d in t h e bladder. As a consequence, due to their greater distance from the bladder, the m a l e gonads receive considerably less dosage t h a n the female. Several conclusions can be d r a w n from the studies. (1) T h e single injection m e t h o d provides a simple a n d reliable alternative to the s t a n d a r d clearance technique for m e a surement of the g l o m e r u l a r filtration rate and a n effective renal p l a s m a flow. (2) T h e a m o u n t of r a d i a t i o n to the target organs, a n d in p a r t i c u l a r to the gonads, is low a n d can be kept so by a d e q u a t e l y h y d r a t i n g the patients a n d instructing t h e m to void frequently. (3) W h e r e v e r the chemical d e t e r m i n a t i o n of inulin can be p e r f o r m e d accurately, the disa p p e a r a n c e of this substance from the blood can be used successfully for the estimation of the g l o m e r u l a r filtration rate, obviating entirely the h a z a r d of radiation.
The ]ournal o[ Pediatrics November 1973
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Measuring glomerular filtration and renal plasma flow
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