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

Abstract

Study Objective: We compare the test characteristics of urine dipstick and urinalysis at various test cutoff points in women presenting to emergency departments and an intermediate care center with symptoms of urinary tract infection. Methods: This was a prospective, observational study of adult women presenting to 1 of 2 community hospital EDs or an intermediate care center with dysuria, urgency, or urinary frequency on history, or suprapubic or costovertebral angle tenderness on examination. Patients who had taken antibiotics in the past 72 hours, had indwelling Foley catheters, symptomatic vaginal discharge, diabetes mellitus, immunodeficiency disorders, or were unable to provide a reliable history were excluded. The patient's clean-catch or catheterized 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 to the hospital laboratory, where a semiautomated microscopic urinalysis and a urine culture were performed. A positive urine culture was defined as more than 100,000 colonies of 1 or 2 uropathogenic bacteria per mL of urine at 48 hours. Dipstick and urinalysis data were compared with urine culture results. Sensitivity, specificity, and predictive values were calculated at various definitions of a positive test, or “test cutoff points,” for combinations of leukocyte esterase, nitrite, and blood on dipstick and for RBCs and WBCs on urinalyses. The probability of an erroneous decision to withhold treatment on the basis of a negative test result was defined as “undertreatment,” or 1 minus the negative predictive value. “Overtreatment” was defined as 1 minus the positive predictive value. Results: Three hundred forty-three patients were enrolled in this study. Twelve patients were withdrawn because of missing laboratory results. Forty-six percent (152/331) of patients had positive urine cultures. If urine dipstick results are defined as positive when leukocyte esterase or nitrite is positive or blood is more than trace, the overtreatment rate is 47% (156/331) and the undertreatment rate is 13% (43/331). If urinalysis results are defined as positive when WBCs are more than 3 per high-power field or RBCs are 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 characteristics 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 undertreatment rates were identified for various test cutoff points for urine dipstick tests and urinalysis. Although a urine dipstick may be equivalent to a urinalysis for the diagnosis of urinary tract infection, the limitations in the diagnostic accuracy of both tests should be incorporated into medical decisionmaking. [Lammers RL, Gibson S, Kovacs D, Sears W, Strachan G. Comparison of test characteristics of urine dipstick and urinalysis at various test cutoff points. Ann Emerg Med. November 2001;38:505-512.]

Introduction

Urinalysis is the most commonly used test for evaluation of emergency department patients with potential urinary tract infections. Unfortunately, in cases in which no other diagnostic studies are needed, urinalysis can significantly increase a patient's time in the ED. Substituting a urine dipstick test for a hospital laboratory urinalysis may be less time-consuming and less expensive, but the dipstick may not be as accurate.

Diagnostic accuracy of urine dipsticks is unclear because methodologies, such as definitions of a positive urine culture and thresholds for test positivity, vary among published studies.1, 2, 3, 4 Blum and Wright¹ used only dipstick leukocyte esterase and nitrite as variables in examining diagnostic performance. Bonnardeaux et al³ studied 5 variables: leukocyte esterase, nitrite, protein, glucose, and ketones. Christenson et al² looked at all 9 variables from the Chemstrip 9 dipstick (Boehringer Mannheim Corporation, Indianapolis, IN). Most studies were conducted in populations that were not typical of ED patients.2, 3, 5 Consequently, some authors recommend confirmatory urinalyses if the urine dipstick results are negative,1, 6 and others if results are positive.2, 3, 6, 7, 8, 9, 10, 11 If the definitions of positive test results, or “test cutoff point,” for urine dipstick and for urinalysis are adjusted, these 2 tests may prove to have comparable sensitivities, specificities, and predictive values at one or more test cutoff points. If this were true, it might be possible to substitute the dipstick test for the urinalysis.

The objective of this study was to compare the test characteristics of urine dipstick and urinalysis at various test cutoff points in women presenting to EDs and an intermediate care center with symptoms of urinary tract infection.