Abstract
Previous studies on diagnostic accuracy of dipstick testing for leukocyte esterase (LE) and nitrite to diagnose urinary tract infection (UTI) had used urine culture, which is an imperfect gold standard. Estimates of diagnostic accuracy obtained using the classical gold standard framework might not reflect the true diagnostic accuracy of dipstick tests. We used the dataset from a prospective, observational study conducted in the emergency department of a teaching hospital in southern India. Patients with a clinical suspicion of UTI underwent dipstick testing for LE and nitrite, urine microscopy, and urine culture. Based on the results of urine microscopy and culture, UTI was classified into definite, probable, and possible. Patients with microscopic pyuria and a positive urine culture were adjudicated as definite UTI. Unequivocal imaging evidence of emphysematous pyelonephritis or perinephric collections was also considered definite UTI. We estimated the diagnostic accuracy of LE and nitrite tests using the classical analysis (assuming definite UTI as gold standard) and two different Bayesian latent class models (LCMs; 3-tests in 1-population and 2-tests in 2-populations models). We studied 149 patients. Overall, 64 (43%) patients had definite, 76 (51%) had probable, and 2 (1.3%) had possible UTI; 7 (4.6%) had alternate diagnoses. In classical analysis, LE was more sensitive than nitrite (87.5% versus 70.5%), while nitrite was more specific (24% versus 58%). The 3-tests in 1-population Bayesian LCM indicated a substantially better sensitivity and specificity for LE (98.1% and 47.6%) and nitrite (88.2% and 97.7%). True sensitivity and specificity of urine culture as estimated by the model was 48.7% and 73.0%. Estimates of the 2-tests in 2-populations model were in agreement with the 3-tests in 1-population model. Bayesian LCMs indicate a clinically important improvement in the true diagnostic accuracy of urine dipstick testing for LE and nitrite. Given this, a negative dipstick LE would rule-out UTI, while a positive dipstick nitrite would rule-in UTI in our study setting. True diagnostic accuracy of urine dipstick testing for UTI in various practice settings needs reevaluation using Bayesian LCMs.
Highlights
Urinary tract infection (UTI) is a common clinical condition encountered among patients seen in various clinical settings including the emergency department (ED)
A loopful of uncentrifuged urine sample was plated on blood agar and CLED (Cysteine-lactose-electrolyte-deficient) agar and incubated aerobically at 37 ̊C for 24 hours, and the growth was described in colony-forming units
We considered definite UTI as the gold standard and estimated the accuracy of dipstick to differentiate definite UTI from the remainder
Summary
Urine dipstick testing for leukocyte esterase (LE) and nitrite is often used to aid clinical decision making when a diagnosis of UTI is considered. The diagnostic accuracy of dipstick testing has been found to widely vary between studies. Applying the principles of quality assessment of diagnostic accuracy studies [1], there could be four major reasons behind this variability. The clinical syndrome of UTI encompasses a wide range of clinical severity which might exert a spectrum effect on the diagnostic accuracy of dipstick testing [7]. Most of the studies did not take into account the clinical pre-test probability while interpreting the dipstick results [8]. The utility of urine dipstick testing differs depending on the population tested
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