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Sodium-potassium measurement interferences in various biological fluids
CLINICA
I35
CHIMICA ACTA
SODIUM-POTASSIUM BIOLOGICAL
MEASUREMENT
INTERFERENCES
IN VARIOUS
FLUIDS
R. E. KNAUFF Cystic Fibrosis
Philadelphia, (Received
Research Institute Laboratories, Pennsylvania ~gzjo (U.S.A.)
December
Temple
University
School of Medicine,
18, 1967)
SUMMARY
The filter-photocell flame photometers, operated with low-temperature flames, have come into widespread use for the measurement of sodium and potassium in serum1T2. These instruments function well in this measurement for which they were designed.
Their use for measurements
the sodium/potassium precaution.
with biological
ratio may be markedly
fluids other than serum, where
different,
was found to require
great
This note describes the precautions necessary in the determination of sodium and potassium in mucus (tracheobronchial), duodenal fluid, saliva, and sweat with a commonly used flame photometer. These particular fluids are of interest in investigations of cystic fibrosis.
MATERIALS
AND METHODS
Equipment An Advanced flame photometer (Model IIB; Advanced Instruments, Inc., Newton Highlands, Mass.) was used in this study. Minor modifications of the instrument were: (I) a reference mark was etched on the pour-through funnel 7 mm below its rim, and (2) the panel null meter was provided with scale graduations to the limits of its needle movement. Standards All standards were prepared from the highest grade of salts in deionized distilled water and were stored in polyethylene containers. Stock solutions of sodium, potassium, and lithium chloride were prepared at concentrations of IO, 5, and 800 mequiv. 1 1, respectively. No mixed sodium-potassium standards were used. All standards contained lithium chloride at a concentration of 40 mequiv./l. Procedures The sample is diluted on the basis of its expected
sodium or potassium
Clin. Chim.
Acta,
20 (1968)
content 135-138
136
KNAGFF
to contain less than 5.0 mequiv,jl of sodium or less than 0.40 mequiv./l of potassium. A reading is made of this diluted sample against a 1.40 meyuiv./l sodium or a 0.05 mequiv./l potassium standard. This reading is compared to the calibration of these two standards in an extremely wide range (0.40 to 5.00 mequiv./l sodium; 0.05 to 0.40 mequiv./l potassium). In order to manage this wide range of concentrations on a single reference set, it is necessary to read the null meter scale after the dial rests at o or 1,000. This initial reading serves only to guide a further dilution of the sample into one of the precision quantitation ranges (Table I) on the basis of the sodium/ potassium ratio of the sample. TABLE
I
INSTRUMENT
PROGRAM
FOR
SODIUM
ASD
POTASSIUM
ANALYSES
Obligatory Atomic Ratio
Q~U~t~tUt~o~ RCZnge
Standards _____--.-set
Primary Secondary Tertiary
1.40
1.00-2.00
0.14
0.02-0.30
0.05
>23.3 >0.36
O.OI-0.09
>o.zo
KS.0
Primary
0.05
0.01-0.10
<56
>0.018
(mequh./E) Intervd
_~
Xa/K
---~ K/Na <0.043 <2.8
The appropriately diluted sample (Table I) is read in one of the established quantitation ranges where it is bracketed by standards. When the sample and standards are read, their solutions fill the how-through funnel to the etched reference mark & z mm. No measurement is accepted until the sodiumlpotassium ratio is calculated to verify that the analysis was performed in an appropriate quantitation range. Calculation is by graph. RESULTS
AND DISCUSSIOX
It was essential to define the interference of sodium on potassium and vice versa. Interference measurements are illustrated in Figs. I and 2. Potassium interferes with sodium measurement (Fig. T) to produce a negative error when the sodium/ potassium ratio is (23.3 (K/Na ratio >0.043) and when the readings are made in the range suggested by the instrument manufacturer. If the sample is diluted further, to be read with more dilute sodium standards, the sodium~potassium ratio can be as low as 0.20 (K/Na ratio of 5.0). An intermediate dilution accommodates ratios as low as 0.36 (K/Na ratio of 2.8). Sodium interference with potassium measurement (Fig. zf, which is much less adverse, results in a positive error when the sodium/potassium ratio is >56. The interference characteristics of the instrument are very favorable for serum and rather unfavorable for some other biological samples. If samples like saliva, sweat, and mucus are quantitated for sodium in the usual range of the instrument (i.e., the range suggested by the manufacturer and used for serum measurements), the average sample will be in error to the extent of about 12, IO and 3%, respectively. Approximately one-half of the samples will have errors in excess of these values-some samples will have errors >25%. It was necessary, therefore, to develop alternative quantitation ranges for the sodium detesmination and to impose obligatory dilutions up to C&Z.Chi$+Z. L&&Z, 20 (1968)
135-138
Na/K
fi rr I1c Y
MEASUREMENT
INTERFERENCES
IN BIOLOGICAL FLUIDS
137
-10
-20
T &_5 w
-30
K/Ma
Fig. I. Potassium ion ccmcentrations
0.1
1
10
ATOMIC
0.01
RATIO
interference with sodium measurement. Lines A, B, and C represent sodium of zoo, 0.30, and 0.09 mequiv.jl, respectively,
several-thollsand-fold. When the outlined precautions are applied, the various fluids can be analyzed with accuracy, precision, and recovery comparable to serum. There seems to be a general unawareness about the limitations of filter-photocell instruments. Investigators use them on samples other than serum without essential orientation and recovery studies, Investigators, in the area. of cystic fibrosis
1000
100
Na/K
Fig. 2. Sodium interference tration of 0.10 mequiv./I.
10
ATOMIC
1
RATIO
with potassium
measurement.
Line represents potassium
ion concen-
research for example3sP, publish massive flame photometric data for sodium and potassium in a variety of fluids without any description of instrumentation or methodology.
138
KNAUFF
It is unfortunate and puzzling that this gross technical hazard has not been described previously. Perhaps this is why researchers study the profound defect in sodium metabolism in cystic fibrosis by use of chloride analysess. REFERENCES I J. A. DEAN, Flame Photometry, McGraw Hill, New York, 1960. 2 D. MACINTYRE, inH. SOBOTKAANDC.P.STEWART(E~~.), Advances in ClinicalChemistry, Vol. 4. Academic Press, New York-London, 1961. 3 P. A. DI SANT'AGNESE AND G. F. POWELL, Ann. N. Y. Acad. Sci., 93 (1962) 555. 4 H. SHWACHMAN AND I. ANTONOWICZ. Ann. N. Y. Acad. Sci., g3 (1962) 600. 5 F. A. IBBOTT, in S. MEITES (Ed.),Standard Methods qf Clinical Chemistry, Vol. 5, Academic Press, New York-London, 1965. C&z. Chim. Acta,
20 (1968) 135-138
I35
CHIMICA ACTA
SODIUM-POTASSIUM BIOLOGICAL
MEASUREMENT
INTERFERENCES
IN VARIOUS
FLUIDS
R. E. KNAUFF Cystic Fibrosis
Philadelphia, (Received
Research Institute Laboratories, Pennsylvania ~gzjo (U.S.A.)
December
Temple
University
School of Medicine,
18, 1967)
SUMMARY
The filter-photocell flame photometers, operated with low-temperature flames, have come into widespread use for the measurement of sodium and potassium in serum1T2. These instruments function well in this measurement for which they were designed.
Their use for measurements
the sodium/potassium precaution.
with biological
ratio may be markedly
fluids other than serum, where
different,
was found to require
great
This note describes the precautions necessary in the determination of sodium and potassium in mucus (tracheobronchial), duodenal fluid, saliva, and sweat with a commonly used flame photometer. These particular fluids are of interest in investigations of cystic fibrosis.
MATERIALS
AND METHODS
Equipment An Advanced flame photometer (Model IIB; Advanced Instruments, Inc., Newton Highlands, Mass.) was used in this study. Minor modifications of the instrument were: (I) a reference mark was etched on the pour-through funnel 7 mm below its rim, and (2) the panel null meter was provided with scale graduations to the limits of its needle movement. Standards All standards were prepared from the highest grade of salts in deionized distilled water and were stored in polyethylene containers. Stock solutions of sodium, potassium, and lithium chloride were prepared at concentrations of IO, 5, and 800 mequiv. 1 1, respectively. No mixed sodium-potassium standards were used. All standards contained lithium chloride at a concentration of 40 mequiv./l. Procedures The sample is diluted on the basis of its expected
sodium or potassium
Clin. Chim.
Acta,
20 (1968)
content 135-138
136
KNAGFF
to contain less than 5.0 mequiv,jl of sodium or less than 0.40 mequiv./l of potassium. A reading is made of this diluted sample against a 1.40 meyuiv./l sodium or a 0.05 mequiv./l potassium standard. This reading is compared to the calibration of these two standards in an extremely wide range (0.40 to 5.00 mequiv./l sodium; 0.05 to 0.40 mequiv./l potassium). In order to manage this wide range of concentrations on a single reference set, it is necessary to read the null meter scale after the dial rests at o or 1,000. This initial reading serves only to guide a further dilution of the sample into one of the precision quantitation ranges (Table I) on the basis of the sodium/ potassium ratio of the sample. TABLE
I
INSTRUMENT
PROGRAM
FOR
SODIUM
ASD
POTASSIUM
ANALYSES
Obligatory Atomic Ratio
Q~U~t~tUt~o~ RCZnge
Standards _____--.-set
Primary Secondary Tertiary
1.40
1.00-2.00
0.14
0.02-0.30
0.05
>23.3 >0.36
O.OI-0.09
>o.zo
KS.0
Primary
0.05
0.01-0.10
<56
>0.018
(mequh./E) Intervd
_~
Xa/K
---~ K/Na <0.043 <2.8
The appropriately diluted sample (Table I) is read in one of the established quantitation ranges where it is bracketed by standards. When the sample and standards are read, their solutions fill the how-through funnel to the etched reference mark & z mm. No measurement is accepted until the sodiumlpotassium ratio is calculated to verify that the analysis was performed in an appropriate quantitation range. Calculation is by graph. RESULTS
AND DISCUSSIOX
It was essential to define the interference of sodium on potassium and vice versa. Interference measurements are illustrated in Figs. I and 2. Potassium interferes with sodium measurement (Fig. T) to produce a negative error when the sodium/ potassium ratio is (23.3 (K/Na ratio >0.043) and when the readings are made in the range suggested by the instrument manufacturer. If the sample is diluted further, to be read with more dilute sodium standards, the sodium~potassium ratio can be as low as 0.20 (K/Na ratio of 5.0). An intermediate dilution accommodates ratios as low as 0.36 (K/Na ratio of 2.8). Sodium interference with potassium measurement (Fig. zf, which is much less adverse, results in a positive error when the sodium/potassium ratio is >56. The interference characteristics of the instrument are very favorable for serum and rather unfavorable for some other biological samples. If samples like saliva, sweat, and mucus are quantitated for sodium in the usual range of the instrument (i.e., the range suggested by the manufacturer and used for serum measurements), the average sample will be in error to the extent of about 12, IO and 3%, respectively. Approximately one-half of the samples will have errors in excess of these values-some samples will have errors >25%. It was necessary, therefore, to develop alternative quantitation ranges for the sodium detesmination and to impose obligatory dilutions up to C&Z.Chi$+Z. L&&Z, 20 (1968)
135-138
Na/K
fi rr I1c Y
MEASUREMENT
INTERFERENCES
IN BIOLOGICAL FLUIDS
137
-10
-20
T &_5 w
-30
K/Ma
Fig. I. Potassium ion ccmcentrations
0.1
1
10
ATOMIC
0.01
RATIO
interference with sodium measurement. Lines A, B, and C represent sodium of zoo, 0.30, and 0.09 mequiv.jl, respectively,
several-thollsand-fold. When the outlined precautions are applied, the various fluids can be analyzed with accuracy, precision, and recovery comparable to serum. There seems to be a general unawareness about the limitations of filter-photocell instruments. Investigators use them on samples other than serum without essential orientation and recovery studies, Investigators, in the area. of cystic fibrosis
1000
100
Na/K
Fig. 2. Sodium interference tration of 0.10 mequiv./I.
10
ATOMIC
1
RATIO
with potassium
measurement.
Line represents potassium
ion concen-
research for example3sP, publish massive flame photometric data for sodium and potassium in a variety of fluids without any description of instrumentation or methodology.
138
KNAUFF
It is unfortunate and puzzling that this gross technical hazard has not been described previously. Perhaps this is why researchers study the profound defect in sodium metabolism in cystic fibrosis by use of chloride analysess. REFERENCES I J. A. DEAN, Flame Photometry, McGraw Hill, New York, 1960. 2 D. MACINTYRE, inH. SOBOTKAANDC.P.STEWART(E~~.), Advances in ClinicalChemistry, Vol. 4. Academic Press, New York-London, 1961. 3 P. A. DI SANT'AGNESE AND G. F. POWELL, Ann. N. Y. Acad. Sci., 93 (1962) 555. 4 H. SHWACHMAN AND I. ANTONOWICZ. Ann. N. Y. Acad. Sci., g3 (1962) 600. 5 F. A. IBBOTT, in S. MEITES (Ed.),Standard Methods qf Clinical Chemistry, Vol. 5, Academic Press, New York-London, 1965. C&z. Chim. Acta,
20 (1968) 135-138