Comparison of test characteristics of urine dipstick and urinalysis at various test cutoff points

Study objective: We compare the test characteristics of urine dipstick and urinalysis at various test Cutoff points in women presenting to emergency d epartments and an intermediate care center with symptoms of urinary tract i nfection. Methods: This was a prospective, observational study of adult women present ing to 1 of 2 community hospital EDs or an intermediate care center with dy suria, urgency, or urinary frequency on history, or suprapubic or costovert ebral angle tenderness on examination. Patients who had taken antibiotics i n the past 72 hours, had indwelling Foley catheters, symptomatic vaginal di scharge, diabetes mellitus, immunodeficiency disorders, or were unable to p rovide a reliable history were excluded. The patient's clean-catch or cathe terized urine specimen was tested immediately by a nurse using a Multistix 9 SG reagent strip. A second aliquot was sent within 1 hour of collection t o the hospital laboratory, where a semiautomated microscopic urinalysis and a urine culture were performed. A positive urine culture was defined as mo re than 100,000 colonies of 1 or 2 uropathogenic bacteria per mL of urine a t 48 hours. Dipstick and urinalysis data were compared with urine culture r esults. Sensitivity, specificity, and predictive values were calculated at various definitions of a positive test, or "test cutoff points," for combin ations of leukocyte esterase nitrite and blood on dipstick and,, for RBCs a nd WBCs on urinalyses. The probability of an erroneous decision to withhold treatment on the basis of a negative test result was defined as "undertrea tment," or 1 minus the negative predictive value. "Overtreatment" was defin ed as 1 minus the positive predictive value. Results: Three hundred forty-three patients were enrolled in this study. Tw elve patients were withdrawn because of missing laboratory results. Forty-s ix percent (152/331) of patients had positive urine cultures. If urine dips tick results are defined as positive when leukocyte esterase or nitrite is positive or blood is more than trace, the overtreatment rate is 47% (156/33 1) and the undertreatment rate is 13% (43/331). If urinalysis results are d efined as positive when WBCs are more than 3 per high-power field or RBCs a re more than 5 per high-power field, the overtreatment rate is 44% (146/331 ) and the undertreatment rate is 11% (36/331). Matched pairs of test charac teristics were identified when the analysis was repeated using more than 10 ,000 colonies per mL as a positive culture. Conclusion: In this patient population, similar overtreatment and undertrea tment rates were identified for various test cutoff points for urine dipsti ck tests and urinalysis. Although a urine dipstick may be equivalent to a u rinalysis for the diagnosis of urinary tract infection, the limitations in the diagnostic accuracy of both tests should be incorporated into medical d ecisionmaking.