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Clinical Chemistry, Vol 32, 1400-1402, Copyright © 1986 by American Association for Clinical Chemistry
JH Pendergrass, AC Skjold, JA Cattell and LR Stover
The control system described here is used to monitor the performance of urinary reagent-strip tests for leukocytes. The presence of leukocyte esterase in the urine is used as a marker for leukocytes in urine. The control system is based on sonicated leukocytes, isolated from whole blood. The esterase activity of this sonicate is determined by spectrophotometry with the N-tosyl-L-alanine ester of 5-phenyl-3- hydroxypyrrole as substrate. The assay result is used to determine the amount of sonicate needed to prepare buffered esterase-containing solutions. Such control solutions mimic leukocytic urines and are stable for 6 h at room temperature. The variability of the control system was tested by preparing it five times in a day on five separate days. The overall CV for Leukostix Reagent Strips (Ames Division, Miles Laboratories, Inc.) when tested with these solutions and analyzed with a reflectance spectrophotometer was 8%; for visual readings it was 10%. The overall CV for Chemstrip LN Reagent Strips (Biodynamics, Indianapolis, IN) was 10%.
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