| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Letters |
Clinical Chemistry Service, Clinical Pathology Department, Warren G. Magnuson Clinical Center, National Institutes of Health, Bldg. 10, Rm. 2C-407, Bethesda, MD 20892-1508, Fax 301-402-1885, E-mail gcsako{at}nih.gov
To the Editor:
Urine additives/preservatives (1) and drugs excreted in the urine (2) can interfere in urine test strip (dipstick) methods. The uroprotective drug mesna (sodium 2-mercaptoethane sulfonate) and other free-sulfhydryl compounds produce false-positive results in ketone methods that are based on the Legal reaction (alkaline sodium nitroprusside) (3). I observed that these compounds also cause false-positive results for ascorbic acid with the Rapignost® Total Screen L dipstick (Behring Diagnostics).
For quantitative assessment of the phenomenon, urine specimens were pooled to contain various concentrations of ascorbic acid, glucose, protein, and/or red blood cells/hemoglobin. When these pools were tested for ascorbic acid with Rapignost (visual reading) and for other analytes with Chemstrip® 10UA (Roche/Boehringer Mannheim) on an automated Chemstrip analyzer, positive reactions included ascorbic acid (+) and protein (0.30 g/L or ++) in pool 1; leukocytes (25–100/µL or trace to +), protein (0.15 g/L or trace), and red blood cells/hemoglobin (25–50 red blood cells/µL or + hemoglobin) in pool 2; and protein (1.00 g/L or ++), glucose (2.5 g/L), and red blood cells/hemoglobin (10–25 red blood cells/µL or trace to + hemoglobin) in pool 3. Increasing concentrations [final concentration, 1.25 mmol/L (0.2 g/L), 2.5 mmol/L (0.4 g/L), and 5.0 mmol/L (0.8 g/L)] of mesna (Mesnex® injection; Asta Medica/Mead Johnson, a Bristol-Meyers Squibb Company) were added to each pool; control pools received physiologic saline instead of mesna. Previously, these concentrations had produced small, moderate, and large amounts of ketone bodies (as false-positive reactions) by visual reading of the Chemstrip 9 dipstick (Roche) (3). Upon duplicate testing with Chemstrip 10UA on the Chemstrip analyzer, mesna did not interfere with detection of leukocytes, protein, glucose, and red blood cells/hemoglobin. Although all three original urine pools were negative for ketones, after mesna was added, they all became positive (++ to +++) for ketones. Testing with the Acetest® tablet method (Ames) disclosed the formerly described false-positive ketone pattern for mesna: a promptly appearing and quickly fading purple violet-magenta color (3). Visual reading of the urine specimens with Rapignost showed development of dose-dependent false-positive ketone (+ to ++) and false-positive ascorbic acid (+ to ++) reactions in the presence of 1.25–5.0 mmol/L (0.2–0.8 g/L) mesna. In pool 1, mesna obliterated assessment of the true ascorbic acid reaction.
Because the urinary concentration of mesna can be as high as 
10
mmol/L (4)(5), random urine specimens obtained
from patients known to receive intravenous mesna but not ascorbic acid
were analyzed. The presence of mesna in these specimens was suggested
by false-positive ketone reactions with Chemstrip 10UA and Acetest.
When the specimens were tested with Rapignost (visual reading),
false-positive reactions developed in both the ketone and ascorbic acid
pads; furthermore, the intensity of false-positive ketone reactions
directly correlated with the intensity of apparently false-positive
ascorbic acid results (Table 1
).
|
To investigate whether the false-positive ascorbic acid reactions were unique to mesna or were merely related to the presence of free sulfhydryl groups, increasing concentrations (0.1, 1, 4, 6, and 10 mmol/L) of two other free-sulfhydryl compounds, cysteine and homocysteine (both from Sigma Chemical), were added to aliquots of a urine specimen from a healthy subject. Visual testing of these specimens with Rapignost revealed dose-dependent false-positive ascorbic acid and ketone reactions and a gross correlation between the two. The ascorbic acid reaction was only borderline positive at 0.1 mmol/L cysteine and homocysteine, but it became clearly positive at higher concentrations of these compounds (+ at 1 mmol/L and + to ++ at or above 4 mmol/L). In the same specimens, the false-positive ketone reactions caused by cysteine or homocysteine ranged from questionably + to +++ with Rapignost and from questionably + to ++ with Chemstrip 9.
The Rapignost package insert (6) and a recent textbook on urinalysis (7) state no known interferences for ascorbic acid. Likewise, no analytical interferences are listed for ascorbic acid in a comprehensive drug interference book (2). The reagent test pad for ascorbic acid contains Tillman’s reagent (an oxidized dye, 2,6-dichloro-phenol-indophenol) that is reduced by ascorbic acid, causing a gray-blue to orange color shift (8). The free sulfhydryl groups of mesna and other compounds apparently exert a similar reducing effect and produce the same color change as ascorbic acid.
Thus, this "ascorbic acid" test really is a test for strong reducing substance(s). As such, it may not always be reliable for monitoring ascorbic acid excretion, but its positivity helps to identify urine specimens that will likely produce false-negative hemoglobin results with methods similar to the one used in Rapignost (6).
Based on usage and pharmacokinetics, mesna, captopril, and N-acetylcysteine are the most likely sources of false-positive ascorbic acid (and ketone) reactions at the present. Unlike false-positive ketone reactions with free-sulfhydryl compounds (3)(9), false-positive ascorbic acid reactions can be suspected only with relevant medication history and/or concomitant occurrence of false-positive ketone reactions. Although Chemstrip 10UA and related Roche package inserts mention free-sulfhydryl compounds and mesna specifically as possible causes of false-positive reactions, the Rapignost package insert (6) does not list free-sulfhydryl drugs and a recent urinalysis textbook (7) mentions only captopril (without specifying the chemical group involved) as a possible cause of false-positive ketone results in Legal reaction-based dipstick pads. Furthermore, almost all clinical diagnostic laboratories in the US failed to recognize these false-positive ketone reactions in survey specimens (10).
It is also relevant to mention that above-therapeutic concentrations of mesna, which carry a high reducing capacity, did not cause interference in two Chemstrip 10UA tests that are based on oxidation reactions (glucose by glucose oxidase/peroxidase and hemoglobin with its strong pseudoperoxidase activity). This observation is in accordance with the manufacturer’s claim that even high concentrations of ascorbic acid will rarely cause falsely low results for these tests with Chemstrip 9 or 10UA (11).
Acknowledgments
I thank Rene Costello for excellent technical assistance and Christine King for specimen acquisition.
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
