Implications of probability analysis for interpreting results of leukocyte esterase and nitrite test strips.

We examined 288 urine samples, using test strips, sediment microscopy, and culture. The ability of the leukocyte esterase and nitrite test strips to detect or exclude urinary tract infection, as defined by a positive culture, was evaluated by probability analysis. We found that the diagnostic efficiency of the esterase-nitrite combination was similar to that of sediment microscopy. Moreover, once the strip test results had been obtained, little additional information was given by microscopy. The importance of estimating the prevalence, or pre-test probability, of infection before the test result is evaluated is emphasized. We conclude that, for detecting or excluding urinary-tract infection, microscopy can be replaced by the esterase and nitrite test strips. If the probability of infection predicted by the test strips is not high (or low) enough compared with medical decision limits, the samples should be cultured.