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Do urine dipsticks reliably predict microhematuria? The bloody truth!
ORIGINAL CONTRIBUTION microhematuria, detection of; urine dipsticks, testing of
Do Urine Dipsticks Reliably Predict Microhematuria? The Bloody Truth! Some literature has cast doubt on the reliability of urine dipsticks in the search for microhematuria. In our study, 2,000 urine dipsticks (two brands) were tested on urine samples containing 0, 0 to 5, 5 to 10, I0 to 20, and 20 to 50 red blood ceils per high-power field. The same samples were retested four hours later and the same sticks were tested after four months of aging. Variables controlled for included observer, lighting, performance according to package insert, pH, specific gravity, white blood cells, ascorbic acid, protein, age of dipstick, and exposure of urine to povidine or cleaning solutions. Results obtained using both N-Multistix ® and Chemstrip ® 9 urine dipsticks correlated closely with the degree of hematuria. Overall sensitivity was 100%, and specificity was 99.3%. We conclude that these urine dipsticks are reliable and are not affected when tested on urine four hours after void, and that Chemstrips ® 9 are much more reliable than are N-Multistix ®if aged in air for four months. [Moore GP, Robinson M: Do urine dipsticks reliably predict microhematuria? The bloody truth! A n n Emerg M e d March 1988; 17:257-260.]
Gregory P Moore, MD* Tacoma, Washington Mel Robinson, MDt Colorado Springs, Colorado From the Departments of Emergency Medicine, Madigan Army Medical Center, Tacoma, Washington;* and St Francis Hospital, Colorado Springs, Colorado.t Received for publication June 11, 1987. Accepted for publication September 2, 1987. The opinions and assertions contained herein are not to be construed as official or representing the views of the United States Army or St Francis Hospital.
INTRODUCTION Urine dipsticks are widely used in emergency departments, as well as other clinical settings throughout the world. They offer a rapid screening tool for assessment of possible pathology in the face of microhematuria. A critical use of urine dipsticks for hematuria has arisen in the management of blunt trauma. Positive urine dipsticks may mandate further immediate diagnostic tests according to some,] while others advocate a period of observation. 2 A recent study condemned the use of urine dipsticks for screening microhematuria in blunt trauma patients. 3 The authors cited m a n y false-negatives and false-positives in their study of 185 blunt trauma patients, of w h o m 104 had hematuria. With the use of urine dipsticks, m a n y variables can contribute to inaccurate diagnosis of hematuria, and these may not have been controlled in this small, retrospective study. The availability of a rapid, reliable, inexpensive screening test for microhematuria is desirable. We studied two widely used brands of urine dipsticks in large numbers at various amounts of known hematuria. Variables controlled for included interobserver variance, lighting, performance according to package insert, pH, specific gravity, pyuria, ascorbic acid, protein, age of dipstick, and exposure of the urine to povidine or cleaning solutions. The effects of delaying interpretation until four hours after void, as well as using dipsticks exposed to room air for four months, also were studied.
Address for reprints: Gregory P Moore, MD, Department of Emergency Medicine, Fort Hood Residency Program, Killeen, Texas 76544.
MATERIALS A N D METHODS In Part 1, fresh-voided urine was obtained from the author, who had no medical problems, was not on any medications, and had refrained from supplemental vitamin C one week prior to study. The sample was tested by urine dipstick to ensure negative glucose, ketones, and protein, as well as a normal specific gravity. Absence of red or white blood cells was confirmed microscopically. Vitamin C was 0 to 10 mg/dL per dipstick and pH was > 6.5. Sterile, unused, unwashed materials were used. Blood was drawn from the author and added to the urine to give known concentrations of hematuria. Methodology was based on work by Vaughan, who added 10 l a m b d a of blood (HCT 43) to 10 mL of urine. This would give 150 red blood cells (RBC)/ 17:3 March 1988
Annals of Emergency Medicine
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D I P S i ~' ,KS & M I C R O H E M A T U R I A Moore & Robinson
high-power field (hpf) after spinning five minutes at 2,000 revolutions per minute (RPM) and then resuspending the bottom 1 mL of the container. 4 Dilutions were made to give concentrations of 0, 0 to 5, 5 to 10, 10 to 20, and 20 to 50 RBC/hpf. These were confirmed by c o u n t i n g t e n fields randomly under the microscope. Forty urine dipsticks of two different brands were randomly tested by an intensive care u n i t nurse and a non-health worker with normal vision who were blinded to concentrations. Urine dipsticks provided by the manufacturers were as follows: N-Multistix ® (14 m o n t h s prior to expiration, lot number Al18046, Ames Division, Miles Laboratories, Elkhart, Indiana) and Chernstrip ® 9 (lot number 250027 26 months prior to expiration, Boehringer M a n n h e i m Diagnostics, Indianapolis, Indiana). Trials were done after education according to package inserts. Specifically, the urine dipsticks were stored below 30 C, dipped for one second with excess urine removed, and compared in horizontal position w i t h a color chart with good lighting. Blood components of the urine dipsticks were read at 50 seconds after dipping with the N-Multistix ® and at 60 seconds w i t h the C h e m s t r i p ® 9. Urine was gently mixed prior to testing to prevent the settling of red cells (Tables 1
TABLE 1. Chemstrip ® 9 results using fresh urine Dipstick Reading RBC/hpf
Negative
5-10 1
4). T h e u r i n e dipsticks were stored with the lids off in a dark area at room temperature for four months. The procedure in Part 1 was performed on these "aged" dipsticks (Table 5). RESULTS The results are shown (Tables 1-5). The urine dipstick readings are plotted quantitatively against the known RBC/hpf in each solution. For N-Mult i s t i x ®, t h e c o l o r r e a d i n g s corresponded to labels of negative, trace, small, moderate, and large. For Chemstrip ® 9, t h e color readings corresponded to labels of negative, 5 to 10, 50, and 250 RBC/hpf. In all trials the amount of hematuria c o r r e l a t e d w i t h an i n c r e a s i n g positivity of urine dipstick reading to a high degree. The accuracy was not 130/258
250
--
--
79
0-5
--
5-10
--
--
10-20
--
--
20-50
--
--
--
Total
79
13
132
12
67 65 --
Total 80
1
80
15
80
80
80
80
80
176
400
TABLE 2. N-Multistix ® results using fresh urine Dipstick Reading RBC/hpf 0
Negative
Trace
Small
Moderate
Large
78
2
--
--
--
80
--
--
80
0-5
--
--
80
5-10
--
--
8
10-20
.
.
.
20-50
.
.
.
Total
78
2
53
Total
19
80
.
80
80
.
80
80
179
400
88
53
TABLE 3. Chemstrip ® 9 results using urine four hours after void
and 2). In Part 2, the same specimens were stored at r o o m temperature for four hours. The procedure then was repeated with the same conditions, observers, and urine dipsticks (Tables 3 and
50
0
Dipstick Reading RBC/hpf 0
Negative
5-10
50
80
--
--
0-5
--
80
5-10
.--
72
10-20
~-
20-50 Total
80
affected by letting the urine sit for four hours. When the tops were left off the urine dipsticks (as may happen in actual ED settings) for four months, the Chemstrips ® 9 maintained a high accuracy, but the N-Multistix ® were grossly positive to the naked eye before being dipped in 98% of the instances. The manufacturer warns that the results are not reliable more than two m o n t h s after the lid has been opened initially. Overall, the s e n s i t i v i t y of b o t h brands of urine dipsticks was 100% and specificity was 99.3%, excluding Annals of Emergency Medicine
E
--
--
0
152
250 = --
Total 80
--
80 8
80
80
80
80
80
168
400
the N - M u l t i s t i x ® f o u r - m o n t h tria~l. There were no fals'e~negatives in 2,000 trials. N-Multistix ® correlated degree of h e m a t u r i a w i t h u r i n e d i p s t i c k positivity per Kendall's Tau test result of 0.90 on fresh urine and 0.92 on four hours post-void urine (P < .01). Ghemstrip ® 9 correlated degree of hematuria w i t h u r i n e dipsticks p o s i t i v i t y per Kendall's Tan test result of 0.87 on fresh u r i n e and 0.88 on four hours post-void urine (P < .01). The Chemstrip ® 9 reading seemed to be one order of magnitude more positive t h a n indicated by the dip17:3 March 1988
TABLE 4. N-Multistix ® results using urine four hours after void
RBC/hpf
Negative
0
80
0-5
--
5-10
--
10-20
.
20-50
--
Total
80
Trace .
Dipstick Reading Small Moderate .
.
1
--
--
80
74
-
80
6 .
.
--
80
80
6
74
80
80
154
400
--
1
Total 80
79
-.
Large
.
85
TABLE 5. Chemstrip ® 9 results using dipsticks aged in air four months
RBC/hpf
Negative
Dipstick Reading 5-10 50
250 --
Total
0
80
--
--
O-5
--
--
74
6
80 8O
5-10
--
--
14
66
80
10-20
--
--
2
78
80
20-50
--
--
--
80
80
Total
80
--
90
230
400
O n l y n i n e of 4 3 0 N - M u l t i s t i x ® w e r e n o r m a l to t h e n a k e d eye a f t e r a g i n g f o u r m o n t h s .
stick. For example, most readings on the 0 to 10 RBC/hpf specimens were "50." Because there was no interobserver variation, data were grouped.
DISCUSSION Use of urine dipsticks in the diagnosis of m i c r o h e m a t u r i a has been very p o p u l a r and r e p o r t e d as reliable.S-9 In t h e e m e r g e n c y setting, urine dipsticks may be used for suspicion and diagnosis of trauma, infarction, tumor, hemorrhagic disease, toxic drugs, c h e m i c a l s , c a l c u l i t and infections.lO Other studies have recorded significant false-negatives and false-positives.a, llA 2 These m a y have suffered from admitted interobserver variation,~2 use of f r o z e n u n f r e s h urine, 1l o r lack of c o n t r o l of variables. 3 The manufacturers of Chemstrip ® 9 claim a specificity of 100% and sensitivity of 90% at levels of 5 erythrocytes/hpf.S We found an overall s e n s i t i v i t y of 100%, specificity of 99.3%, and no false-negatives in 2,000 trials. Several authors have questioned the use of microscopic analysis of urine as the standard of comparison as there are m a n y sources of error in this tech17:3 M a r c h 1 9 8 8
nique.4,8,t0,13,14 Urine dipsticks utilize the peroxidase activity of hemoglobin to catalyze chemical reactions that cause a color change to green or blue in varying shades. With Chemstrip® 9 the substances involved are tetramethyl-benzidine and 2,5-dimethyl2,5-dihydroperoxyhexane, while N-Multistix ® uses c u m e n e hydroperoxide with 3,3', 5,5'-tetramethylbenzidine. The urine dipsticks are more sensitive to hemoglobin than are intact erythrocytes at a level of 0.015 to 0.062 mg/ dL concentration.S,6,11 Vaughan showed that when there is low urine osmolality (as m a y be expected in trauma patients aggressively resuscitated with crystalloid), red cells lyse and the free hemoglobin makes the urine dipstick more sensitive than the microscopic examination with decreased cells apparent. 4 Up to 50% of cells m a y lyse in 2V2 hours in warm urine, again making the urine dipstick more sensitive than microscopic examination. 8 Leonards stated that microscopic e x a m i n a t i o n "missed" hem a t u r i a 12% of t h e t i m e due to hemolysis. He also stressed the laboratory error of "positive" microscopic identification made by inexperienced
Annals of Emergency Medicine
examiners e r r o n e o u s l y i d e n t i f y i n g WBCs or yeast cells as RBCs. lo Goldner concluded that urine dipsticks are an unsatisfactory screening test and that microscopic examination may be a better predictor of urinary tract injury; he reported a patient with gross hematuria and extravasation on intravenous pyelogram whose microscopic analysis showed 2 to 5 RBC/ hpf. 3 Use of the less expensive urine dipstick rather than microscopic examination as a screening test would result in tremendous monetary savings to patients in the United States annually,s There are several recommendations made by the manufacturers to ensure accurate results. These testing protocols must be followed closely if we are to interpret data supplied by urine dipsticks reliably in clinical or laboratory studies. The manufacturers state that urine dipsticks should be stored at temperatures less than 30 C but not in a refrigerator and out of direct sunlight. The container should be kept dry by immediately capping after use, and not removing the dessicant. The makers of N - M u l t i s t i x ® specifically state that once the vial is opened their urine dipsticks have a t w o - m o n t h span of reliability. The urine sample should be fresh, u n c e n t r i f u g e d , and w i t h o u t preservatives. The urine dipsticks should be dipped one second, excess urine removed, and the sticks held in horizontal position to prevent running of reagents. The blood component should be read at 60 seconds for Chemstrip ® 9 and 50 seconds for N-Multistix®.5, 6 Failure to adhere to these recommendations m a y result in false-negative and f a l s e - p o s i t i v e r e s u l t s in b o t h clinical and research settings. There are several causes of falsepositives. Menstruating women, damage f r o m catheterization, and myoglobinuria are three relatively comm o n possibilities.5-7,1sA 6 High concentrations of local betadine can cause false-positives but not at the levels of exposure in clinical situationsJ s Peroxidases produced by bacteria in urinary infection or urine that has stood too long are other causes.S,6A s Our data showed the latter not to be significant after four hours. Cleaning solutions may give false-positive or inaccurate results, so it is important to avoid contaminated glassware.S,6,9, is In Smith's study, adding 100 mg/L con259/131
DIPSTICKS & MICROHEMATURIA Moore & Robinson
c e n t r a t i o n of s o d i u m hy-pochlorite to u r i n e gave a 2 + u r i n e d i p s t i c k reading i n t h e a b s e n c e of h e m a t u r i a . 9 A m o i s t e n e d or aged u r i n e dipstick can create false-positives.S,6,15 In our study, we found that m o i s t u r e a n d a g i n g s i g n i f i c a n t l y affected t h e N - M u l t i s t i x ®. T h i s had n o t b e e n r e p o r t e d previously. A f t e r controlling for t h e variables, we found a 1% false-positive occurrence. T h e r e are also m a n y causes of falsenegatives in u s i n g u r i n e dipsticks for hematuria diagnosis. High urinary specific gravity, osmolality, or p r o t e i n can a c c o u n t for n e g a t i v e results.4-6, is Crenated, s h r u n k e n RBCs are n o t as easily t a k e n up and lysed by t h e u r i n e dipstick. T h i s h i g h o s m o l a l i t y state in u r i n e m a y be s i g n i f i c a n t in a b l u n t t r a u m a p a t i e n t w h o is in shock. Form a l i n or u r i n a r y p r e s e r v a t i v e s m a y r e n d e r u r i n e d i p s t i c k s n e g a t i v e , as m a y gentisic acid and h i g h u r i n a r y nitrites.S,6,15 W h e n u r i n a r y p H is < 6.0, false-negative u r i n e dipsticks readings are e n c o u n t e r e d J g U r i n e s h o u l d be w e l l m i x e d prior to use of u r i n e dipsticks. If o n l y t h e sup e r f i c i a l s a m p l e is d i p p e d , s e t t l e d RBCs at t h e b o t t o m of t h e c o n t a i n e r are n o t e n c o u n t e r e d . V i t a m i n C can s i g n i f i c a n t l y a c c o u n t for f a l s e - n e g a tives. T h i s can occur at levels > 5 m g / dL.S,6, t5 S m i t h s h o w e d t h a t d e s p i t e 400 R B C / h p f in urine, w h e n 500 m g / L ascorbic acid was present, t h e u r i n e d i p s t i c k was r e d u c e d f r o m a 3 + to 2 + reading. 9 D a a e f o u n d i n h i b i t i o n of u r i n e dipsticks at levels of 0.6 m m o l / L of ascorbic acid in urine. T h i s m a y be present in up to 25% of people in a h e a l t h - c o n s c i o u s society3 7 T h e u r i n e in this sudy was 0 to 10 m g / d L by dipstick for v i t a m i n C - - in o t h e r words, n e g a t i v e by dipstick. We had no false-negatives in 1,600 trials. Interobserver variation can account for b o t h h i g h f a l s e - n e g a t i v e or falsepositive rates, as s h o w n by Asberg. ~2 We h a v e s h o w n in a w e l l - c o n t r o l l e d
132/260
study t h a t u r i n e dipsticks are reliable predictors of urinary m i c r o h e m a t u r i a . We a d v o c a t e t h e d e p e n d e n c e on u r i n e d i p s t i c k s in s c r e e n i n g for h e m a t u r i a . R e c o g n i z i n g t h a t the patient m a y be on v i t a m i n C, in a h i g h or l o w osm o l a l i t y state, and t h a t the u r i n e dipstick is u s e d in a t i m e l y correct m a n ner w i l l eradicate false-negatives. It is true t h a t b l u n t t r a u m a v i c t i m s m a y have m y o g l o b i n u r i a t h a t w o u l d give a false-positive.S, 6 T h e s u b s e q u e n t abs e n c e of R B C s on u r i n a l y s i s w o u l d suggest t h i s diagnosis, again m a k i n g urine dipsticks data v e r y useful. T h e f a l s e - n e g a t i v e r e s u l t is w h a t m u s t be avoided. We had n o n e in our study; our data are supported by Mariani, w h o had a false-negative rate of 0.9% in 1,346 trials. 8 Of 5,000 people in one study, 21.6% had e r y t h r o c y t e s in t h e i r u r i n e w i t h o u t t r a u m a , and 2% h a d m o r e t h a n 10 R B C / h p f 3 8 If t r a u m a t i z e d , t h e s e " h e m a t u r i c " people may have a normal intravenous p y e l o g r a m . A s in o t h e r a s p e c t s of medicine, clinical correlation and p r o p e r u s e of d i a g n o s t i c a i d s m a y e n h a n c e t h e accuracy of evaluation. CONCLUSION Both N - M u l t i s t i x ® and C h e m s t r i p ® 9 u r i n e d i p s t i c k s are e x t r e m e l y reliable in t h e d e t e c t i o n of m i c r o h e m a t u ria. W h e n u r i n e is a l l o w e d to s t a n d four hours before testing, the urine dipsticks do n o t s e e m to be affected in i d e n t i f y i n g h e m a t u r i a . If e x p o s e d to air for p r o l o n g e d periods, C h e m s t r i p ® 9 m a i n t a i n s r e l i a b i l i t y in d e t e c t i n g microscopic hematuria while N-Multistix ® does not. The authors express sincere appreciation to Laura Brumley for assistance in preparation of this manuscript.
2. Erik K, Worthington GS: Indications for emergency IVP in blunt abdominal trauma: A reappraisal. J Trauma 1986;26:1086q089. 3. Goldner AP, Mayron R, Ruiz E: Are urine dipsticks reliable indicators of hematuria in blunt trauma patients? Ann Emerg Med 1985; 14:580-582. 4. Vaughan ED, Wyker AW: Effect of osmolality on the evaluation of microscopic hematuria. J Urol 1971;105:709-711. 5. Package Insert Chemstrip ~ 9. Boehringer Mannheim Diagnostics, Indianapolis, Indiana, 1986. 6. Package insert N-Multistix ®. Ames Division, Miles Laboratories, Elkhart, Indiana, 1986. 7. Shaw ST, Poon SY, Wong ET: 'Routine urinalysis' Is the dipstick enough? JAMA 1985;253: 1596-1600. 8. Mariani AJ, Luangphinith S, Loo S, et ah Dipstick chemical urinalysis: An accurate cost-effective screening test. J Urol 1984;132:64-66. 9. Smith BC, Peake MJ, Fraser CG: Urinalysis by use of multi-test reagent strips. Two dipsticks compared. Clin C h e m 2977;23: 2337-2340. I0. Leonards JR: Simple test for hematuria compared with established tests: JAMA 1962; 179:807-808. 11. Bee DE, James GP, Paul KL: Hemoglobinuria and hematuria: Accuracy and precision of laboratory diagnosis. C1in Chem 1979;25:1696-1699. 12. Asberg A, Lehmann EH, Petersen JE: Evaluation of dipsticks for the assessment of microhematuria. Scand J Clin Lab Invest 1984;44: 377-380. 13. Levin K, Engstrom l: Inadequate hemolysis of erythrocytes on reagent strips at low pH causes false negative readings. Clin Chem 1984; 30:1845-1847. 14. Braun J, Straube W: A new rapid test for di agnosing microhematuria compared with results of microscopic examination. Schweiz Med Wochenschr 1975;100:78-87.
15. Niemi TA: Povidine iodine: A cause of fa!se-positive dipstick hematuria? (letter). Ann Emerg Med 1984;13:984-985. 16. Hockberger RS,,Sehwartz .B, Connor J: Hematuria induced by urethral catheterization. Ann Emerg Med 1987;16:550-552. 17. Daae LNW, Jueff A: Ascorbic acid and test strip reactions for haematuria. Scand J Clin Lab Invest 1983;43:267-269.
REFERENCES
l. Griffin WO, Belin RP, Ernst CB: Intravenous pyelography in abdominal trauma. J Trauma 1978;18:387-391.
Annals of Emergency Medicine
18. Freni SC, Heederik G], Hol C: Centrifugation techniques and reagent strips in the assessment of microhaem'aturia. J Clin Pathol 1977; 30:33&340.
17:3 March 1988
Do Urine Dipsticks Reliably Predict Microhematuria? The Bloody Truth! Some literature has cast doubt on the reliability of urine dipsticks in the search for microhematuria. In our study, 2,000 urine dipsticks (two brands) were tested on urine samples containing 0, 0 to 5, 5 to 10, I0 to 20, and 20 to 50 red blood ceils per high-power field. The same samples were retested four hours later and the same sticks were tested after four months of aging. Variables controlled for included observer, lighting, performance according to package insert, pH, specific gravity, white blood cells, ascorbic acid, protein, age of dipstick, and exposure of urine to povidine or cleaning solutions. Results obtained using both N-Multistix ® and Chemstrip ® 9 urine dipsticks correlated closely with the degree of hematuria. Overall sensitivity was 100%, and specificity was 99.3%. We conclude that these urine dipsticks are reliable and are not affected when tested on urine four hours after void, and that Chemstrips ® 9 are much more reliable than are N-Multistix ®if aged in air for four months. [Moore GP, Robinson M: Do urine dipsticks reliably predict microhematuria? The bloody truth! A n n Emerg M e d March 1988; 17:257-260.]
Gregory P Moore, MD* Tacoma, Washington Mel Robinson, MDt Colorado Springs, Colorado From the Departments of Emergency Medicine, Madigan Army Medical Center, Tacoma, Washington;* and St Francis Hospital, Colorado Springs, Colorado.t Received for publication June 11, 1987. Accepted for publication September 2, 1987. The opinions and assertions contained herein are not to be construed as official or representing the views of the United States Army or St Francis Hospital.
INTRODUCTION Urine dipsticks are widely used in emergency departments, as well as other clinical settings throughout the world. They offer a rapid screening tool for assessment of possible pathology in the face of microhematuria. A critical use of urine dipsticks for hematuria has arisen in the management of blunt trauma. Positive urine dipsticks may mandate further immediate diagnostic tests according to some,] while others advocate a period of observation. 2 A recent study condemned the use of urine dipsticks for screening microhematuria in blunt trauma patients. 3 The authors cited m a n y false-negatives and false-positives in their study of 185 blunt trauma patients, of w h o m 104 had hematuria. With the use of urine dipsticks, m a n y variables can contribute to inaccurate diagnosis of hematuria, and these may not have been controlled in this small, retrospective study. The availability of a rapid, reliable, inexpensive screening test for microhematuria is desirable. We studied two widely used brands of urine dipsticks in large numbers at various amounts of known hematuria. Variables controlled for included interobserver variance, lighting, performance according to package insert, pH, specific gravity, pyuria, ascorbic acid, protein, age of dipstick, and exposure of the urine to povidine or cleaning solutions. The effects of delaying interpretation until four hours after void, as well as using dipsticks exposed to room air for four months, also were studied.
Address for reprints: Gregory P Moore, MD, Department of Emergency Medicine, Fort Hood Residency Program, Killeen, Texas 76544.
MATERIALS A N D METHODS In Part 1, fresh-voided urine was obtained from the author, who had no medical problems, was not on any medications, and had refrained from supplemental vitamin C one week prior to study. The sample was tested by urine dipstick to ensure negative glucose, ketones, and protein, as well as a normal specific gravity. Absence of red or white blood cells was confirmed microscopically. Vitamin C was 0 to 10 mg/dL per dipstick and pH was > 6.5. Sterile, unused, unwashed materials were used. Blood was drawn from the author and added to the urine to give known concentrations of hematuria. Methodology was based on work by Vaughan, who added 10 l a m b d a of blood (HCT 43) to 10 mL of urine. This would give 150 red blood cells (RBC)/ 17:3 March 1988
Annals of Emergency Medicine
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D I P S i ~' ,KS & M I C R O H E M A T U R I A Moore & Robinson
high-power field (hpf) after spinning five minutes at 2,000 revolutions per minute (RPM) and then resuspending the bottom 1 mL of the container. 4 Dilutions were made to give concentrations of 0, 0 to 5, 5 to 10, 10 to 20, and 20 to 50 RBC/hpf. These were confirmed by c o u n t i n g t e n fields randomly under the microscope. Forty urine dipsticks of two different brands were randomly tested by an intensive care u n i t nurse and a non-health worker with normal vision who were blinded to concentrations. Urine dipsticks provided by the manufacturers were as follows: N-Multistix ® (14 m o n t h s prior to expiration, lot number Al18046, Ames Division, Miles Laboratories, Elkhart, Indiana) and Chernstrip ® 9 (lot number 250027 26 months prior to expiration, Boehringer M a n n h e i m Diagnostics, Indianapolis, Indiana). Trials were done after education according to package inserts. Specifically, the urine dipsticks were stored below 30 C, dipped for one second with excess urine removed, and compared in horizontal position w i t h a color chart with good lighting. Blood components of the urine dipsticks were read at 50 seconds after dipping with the N-Multistix ® and at 60 seconds w i t h the C h e m s t r i p ® 9. Urine was gently mixed prior to testing to prevent the settling of red cells (Tables 1
TABLE 1. Chemstrip ® 9 results using fresh urine Dipstick Reading RBC/hpf
Negative
5-10 1
4). T h e u r i n e dipsticks were stored with the lids off in a dark area at room temperature for four months. The procedure in Part 1 was performed on these "aged" dipsticks (Table 5). RESULTS The results are shown (Tables 1-5). The urine dipstick readings are plotted quantitatively against the known RBC/hpf in each solution. For N-Mult i s t i x ®, t h e c o l o r r e a d i n g s corresponded to labels of negative, trace, small, moderate, and large. For Chemstrip ® 9, t h e color readings corresponded to labels of negative, 5 to 10, 50, and 250 RBC/hpf. In all trials the amount of hematuria c o r r e l a t e d w i t h an i n c r e a s i n g positivity of urine dipstick reading to a high degree. The accuracy was not 130/258
250
--
--
79
0-5
--
5-10
--
--
10-20
--
--
20-50
--
--
--
Total
79
13
132
12
67 65 --
Total 80
1
80
15
80
80
80
80
80
176
400
TABLE 2. N-Multistix ® results using fresh urine Dipstick Reading RBC/hpf 0
Negative
Trace
Small
Moderate
Large
78
2
--
--
--
80
--
--
80
0-5
--
--
80
5-10
--
--
8
10-20
.
.
.
20-50
.
.
.
Total
78
2
53
Total
19
80
.
80
80
.
80
80
179
400
88
53
TABLE 3. Chemstrip ® 9 results using urine four hours after void
and 2). In Part 2, the same specimens were stored at r o o m temperature for four hours. The procedure then was repeated with the same conditions, observers, and urine dipsticks (Tables 3 and
50
0
Dipstick Reading RBC/hpf 0
Negative
5-10
50
80
--
--
0-5
--
80
5-10
.--
72
10-20
~-
20-50 Total
80
affected by letting the urine sit for four hours. When the tops were left off the urine dipsticks (as may happen in actual ED settings) for four months, the Chemstrips ® 9 maintained a high accuracy, but the N-Multistix ® were grossly positive to the naked eye before being dipped in 98% of the instances. The manufacturer warns that the results are not reliable more than two m o n t h s after the lid has been opened initially. Overall, the s e n s i t i v i t y of b o t h brands of urine dipsticks was 100% and specificity was 99.3%, excluding Annals of Emergency Medicine
E
--
--
0
152
250 = --
Total 80
--
80 8
80
80
80
80
80
168
400
the N - M u l t i s t i x ® f o u r - m o n t h tria~l. There were no fals'e~negatives in 2,000 trials. N-Multistix ® correlated degree of h e m a t u r i a w i t h u r i n e d i p s t i c k positivity per Kendall's Tau test result of 0.90 on fresh urine and 0.92 on four hours post-void urine (P < .01). Ghemstrip ® 9 correlated degree of hematuria w i t h u r i n e dipsticks p o s i t i v i t y per Kendall's Tan test result of 0.87 on fresh u r i n e and 0.88 on four hours post-void urine (P < .01). The Chemstrip ® 9 reading seemed to be one order of magnitude more positive t h a n indicated by the dip17:3 March 1988
TABLE 4. N-Multistix ® results using urine four hours after void
RBC/hpf
Negative
0
80
0-5
--
5-10
--
10-20
.
20-50
--
Total
80
Trace .
Dipstick Reading Small Moderate .
.
1
--
--
80
74
-
80
6 .
.
--
80
80
6
74
80
80
154
400
--
1
Total 80
79
-.
Large
.
85
TABLE 5. Chemstrip ® 9 results using dipsticks aged in air four months
RBC/hpf
Negative
Dipstick Reading 5-10 50
250 --
Total
0
80
--
--
O-5
--
--
74
6
80 8O
5-10
--
--
14
66
80
10-20
--
--
2
78
80
20-50
--
--
--
80
80
Total
80
--
90
230
400
O n l y n i n e of 4 3 0 N - M u l t i s t i x ® w e r e n o r m a l to t h e n a k e d eye a f t e r a g i n g f o u r m o n t h s .
stick. For example, most readings on the 0 to 10 RBC/hpf specimens were "50." Because there was no interobserver variation, data were grouped.
DISCUSSION Use of urine dipsticks in the diagnosis of m i c r o h e m a t u r i a has been very p o p u l a r and r e p o r t e d as reliable.S-9 In t h e e m e r g e n c y setting, urine dipsticks may be used for suspicion and diagnosis of trauma, infarction, tumor, hemorrhagic disease, toxic drugs, c h e m i c a l s , c a l c u l i t and infections.lO Other studies have recorded significant false-negatives and false-positives.a, llA 2 These m a y have suffered from admitted interobserver variation,~2 use of f r o z e n u n f r e s h urine, 1l o r lack of c o n t r o l of variables. 3 The manufacturers of Chemstrip ® 9 claim a specificity of 100% and sensitivity of 90% at levels of 5 erythrocytes/hpf.S We found an overall s e n s i t i v i t y of 100%, specificity of 99.3%, and no false-negatives in 2,000 trials. Several authors have questioned the use of microscopic analysis of urine as the standard of comparison as there are m a n y sources of error in this tech17:3 M a r c h 1 9 8 8
nique.4,8,t0,13,14 Urine dipsticks utilize the peroxidase activity of hemoglobin to catalyze chemical reactions that cause a color change to green or blue in varying shades. With Chemstrip® 9 the substances involved are tetramethyl-benzidine and 2,5-dimethyl2,5-dihydroperoxyhexane, while N-Multistix ® uses c u m e n e hydroperoxide with 3,3', 5,5'-tetramethylbenzidine. The urine dipsticks are more sensitive to hemoglobin than are intact erythrocytes at a level of 0.015 to 0.062 mg/ dL concentration.S,6,11 Vaughan showed that when there is low urine osmolality (as m a y be expected in trauma patients aggressively resuscitated with crystalloid), red cells lyse and the free hemoglobin makes the urine dipstick more sensitive than the microscopic examination with decreased cells apparent. 4 Up to 50% of cells m a y lyse in 2V2 hours in warm urine, again making the urine dipstick more sensitive than microscopic examination. 8 Leonards stated that microscopic e x a m i n a t i o n "missed" hem a t u r i a 12% of t h e t i m e due to hemolysis. He also stressed the laboratory error of "positive" microscopic identification made by inexperienced
Annals of Emergency Medicine
examiners e r r o n e o u s l y i d e n t i f y i n g WBCs or yeast cells as RBCs. lo Goldner concluded that urine dipsticks are an unsatisfactory screening test and that microscopic examination may be a better predictor of urinary tract injury; he reported a patient with gross hematuria and extravasation on intravenous pyelogram whose microscopic analysis showed 2 to 5 RBC/ hpf. 3 Use of the less expensive urine dipstick rather than microscopic examination as a screening test would result in tremendous monetary savings to patients in the United States annually,s There are several recommendations made by the manufacturers to ensure accurate results. These testing protocols must be followed closely if we are to interpret data supplied by urine dipsticks reliably in clinical or laboratory studies. The manufacturers state that urine dipsticks should be stored at temperatures less than 30 C but not in a refrigerator and out of direct sunlight. The container should be kept dry by immediately capping after use, and not removing the dessicant. The makers of N - M u l t i s t i x ® specifically state that once the vial is opened their urine dipsticks have a t w o - m o n t h span of reliability. The urine sample should be fresh, u n c e n t r i f u g e d , and w i t h o u t preservatives. The urine dipsticks should be dipped one second, excess urine removed, and the sticks held in horizontal position to prevent running of reagents. The blood component should be read at 60 seconds for Chemstrip ® 9 and 50 seconds for N-Multistix®.5, 6 Failure to adhere to these recommendations m a y result in false-negative and f a l s e - p o s i t i v e r e s u l t s in b o t h clinical and research settings. There are several causes of falsepositives. Menstruating women, damage f r o m catheterization, and myoglobinuria are three relatively comm o n possibilities.5-7,1sA 6 High concentrations of local betadine can cause false-positives but not at the levels of exposure in clinical situationsJ s Peroxidases produced by bacteria in urinary infection or urine that has stood too long are other causes.S,6A s Our data showed the latter not to be significant after four hours. Cleaning solutions may give false-positive or inaccurate results, so it is important to avoid contaminated glassware.S,6,9, is In Smith's study, adding 100 mg/L con259/131
DIPSTICKS & MICROHEMATURIA Moore & Robinson
c e n t r a t i o n of s o d i u m hy-pochlorite to u r i n e gave a 2 + u r i n e d i p s t i c k reading i n t h e a b s e n c e of h e m a t u r i a . 9 A m o i s t e n e d or aged u r i n e dipstick can create false-positives.S,6,15 In our study, we found that m o i s t u r e a n d a g i n g s i g n i f i c a n t l y affected t h e N - M u l t i s t i x ®. T h i s had n o t b e e n r e p o r t e d previously. A f t e r controlling for t h e variables, we found a 1% false-positive occurrence. T h e r e are also m a n y causes of falsenegatives in u s i n g u r i n e dipsticks for hematuria diagnosis. High urinary specific gravity, osmolality, or p r o t e i n can a c c o u n t for n e g a t i v e results.4-6, is Crenated, s h r u n k e n RBCs are n o t as easily t a k e n up and lysed by t h e u r i n e dipstick. T h i s h i g h o s m o l a l i t y state in u r i n e m a y be s i g n i f i c a n t in a b l u n t t r a u m a p a t i e n t w h o is in shock. Form a l i n or u r i n a r y p r e s e r v a t i v e s m a y r e n d e r u r i n e d i p s t i c k s n e g a t i v e , as m a y gentisic acid and h i g h u r i n a r y nitrites.S,6,15 W h e n u r i n a r y p H is < 6.0, false-negative u r i n e dipsticks readings are e n c o u n t e r e d J g U r i n e s h o u l d be w e l l m i x e d prior to use of u r i n e dipsticks. If o n l y t h e sup e r f i c i a l s a m p l e is d i p p e d , s e t t l e d RBCs at t h e b o t t o m of t h e c o n t a i n e r are n o t e n c o u n t e r e d . V i t a m i n C can s i g n i f i c a n t l y a c c o u n t for f a l s e - n e g a tives. T h i s can occur at levels > 5 m g / dL.S,6, t5 S m i t h s h o w e d t h a t d e s p i t e 400 R B C / h p f in urine, w h e n 500 m g / L ascorbic acid was present, t h e u r i n e d i p s t i c k was r e d u c e d f r o m a 3 + to 2 + reading. 9 D a a e f o u n d i n h i b i t i o n of u r i n e dipsticks at levels of 0.6 m m o l / L of ascorbic acid in urine. T h i s m a y be present in up to 25% of people in a h e a l t h - c o n s c i o u s society3 7 T h e u r i n e in this sudy was 0 to 10 m g / d L by dipstick for v i t a m i n C - - in o t h e r words, n e g a t i v e by dipstick. We had no false-negatives in 1,600 trials. Interobserver variation can account for b o t h h i g h f a l s e - n e g a t i v e or falsepositive rates, as s h o w n by Asberg. ~2 We h a v e s h o w n in a w e l l - c o n t r o l l e d
132/260
study t h a t u r i n e dipsticks are reliable predictors of urinary m i c r o h e m a t u r i a . We a d v o c a t e t h e d e p e n d e n c e on u r i n e d i p s t i c k s in s c r e e n i n g for h e m a t u r i a . R e c o g n i z i n g t h a t the patient m a y be on v i t a m i n C, in a h i g h or l o w osm o l a l i t y state, and t h a t the u r i n e dipstick is u s e d in a t i m e l y correct m a n ner w i l l eradicate false-negatives. It is true t h a t b l u n t t r a u m a v i c t i m s m a y have m y o g l o b i n u r i a t h a t w o u l d give a false-positive.S, 6 T h e s u b s e q u e n t abs e n c e of R B C s on u r i n a l y s i s w o u l d suggest t h i s diagnosis, again m a k i n g urine dipsticks data v e r y useful. T h e f a l s e - n e g a t i v e r e s u l t is w h a t m u s t be avoided. We had n o n e in our study; our data are supported by Mariani, w h o had a false-negative rate of 0.9% in 1,346 trials. 8 Of 5,000 people in one study, 21.6% had e r y t h r o c y t e s in t h e i r u r i n e w i t h o u t t r a u m a , and 2% h a d m o r e t h a n 10 R B C / h p f 3 8 If t r a u m a t i z e d , t h e s e " h e m a t u r i c " people may have a normal intravenous p y e l o g r a m . A s in o t h e r a s p e c t s of medicine, clinical correlation and p r o p e r u s e of d i a g n o s t i c a i d s m a y e n h a n c e t h e accuracy of evaluation. CONCLUSION Both N - M u l t i s t i x ® and C h e m s t r i p ® 9 u r i n e d i p s t i c k s are e x t r e m e l y reliable in t h e d e t e c t i o n of m i c r o h e m a t u ria. W h e n u r i n e is a l l o w e d to s t a n d four hours before testing, the urine dipsticks do n o t s e e m to be affected in i d e n t i f y i n g h e m a t u r i a . If e x p o s e d to air for p r o l o n g e d periods, C h e m s t r i p ® 9 m a i n t a i n s r e l i a b i l i t y in d e t e c t i n g microscopic hematuria while N-Multistix ® does not. The authors express sincere appreciation to Laura Brumley for assistance in preparation of this manuscript.
2. Erik K, Worthington GS: Indications for emergency IVP in blunt abdominal trauma: A reappraisal. J Trauma 1986;26:1086q089. 3. Goldner AP, Mayron R, Ruiz E: Are urine dipsticks reliable indicators of hematuria in blunt trauma patients? Ann Emerg Med 1985; 14:580-582. 4. Vaughan ED, Wyker AW: Effect of osmolality on the evaluation of microscopic hematuria. J Urol 1971;105:709-711. 5. Package Insert Chemstrip ~ 9. Boehringer Mannheim Diagnostics, Indianapolis, Indiana, 1986. 6. Package insert N-Multistix ®. Ames Division, Miles Laboratories, Elkhart, Indiana, 1986. 7. Shaw ST, Poon SY, Wong ET: 'Routine urinalysis' Is the dipstick enough? JAMA 1985;253: 1596-1600. 8. Mariani AJ, Luangphinith S, Loo S, et ah Dipstick chemical urinalysis: An accurate cost-effective screening test. J Urol 1984;132:64-66. 9. Smith BC, Peake MJ, Fraser CG: Urinalysis by use of multi-test reagent strips. Two dipsticks compared. Clin C h e m 2977;23: 2337-2340. I0. Leonards JR: Simple test for hematuria compared with established tests: JAMA 1962; 179:807-808. 11. Bee DE, James GP, Paul KL: Hemoglobinuria and hematuria: Accuracy and precision of laboratory diagnosis. C1in Chem 1979;25:1696-1699. 12. Asberg A, Lehmann EH, Petersen JE: Evaluation of dipsticks for the assessment of microhematuria. Scand J Clin Lab Invest 1984;44: 377-380. 13. Levin K, Engstrom l: Inadequate hemolysis of erythrocytes on reagent strips at low pH causes false negative readings. Clin Chem 1984; 30:1845-1847. 14. Braun J, Straube W: A new rapid test for di agnosing microhematuria compared with results of microscopic examination. Schweiz Med Wochenschr 1975;100:78-87.
15. Niemi TA: Povidine iodine: A cause of fa!se-positive dipstick hematuria? (letter). Ann Emerg Med 1984;13:984-985. 16. Hockberger RS,,Sehwartz .B, Connor J: Hematuria induced by urethral catheterization. Ann Emerg Med 1987;16:550-552. 17. Daae LNW, Jueff A: Ascorbic acid and test strip reactions for haematuria. Scand J Clin Lab Invest 1983;43:267-269.
REFERENCES
l. Griffin WO, Belin RP, Ernst CB: Intravenous pyelography in abdominal trauma. J Trauma 1978;18:387-391.
Annals of Emergency Medicine
18. Freni SC, Heederik G], Hol C: Centrifugation techniques and reagent strips in the assessment of microhaem'aturia. J Clin Pathol 1977; 30:33&340.
17:3 March 1988