Welcome to Facts on the Ground a new series brought to you by Natan Cramer, Pediatric Emergency Medicine fellow from the University of Pittsburgh. Facts on the Ground is designed to be a concise literature review that helps answer common clinical questions.

The Article

The Importance of Urine Concentration on the Diagnostic Performance of the Urinalysis for Pediatric Urinary Tract Infection Chaudhari, et al. Annals of Emergency Medicine, 2017

Objective

To investigate the relationship of urine concentration (specific gravity) to the diagnostic accuracy of leukocyte esterase or pyuria in identifying urinary tract infections (UTI).

Study Design

Retrospective cross-sectional study of urine results (culture and urinalysis/dipstick) of 14,971 children <13 years old presenting to the ED for concern of UTI. Patients were excluded if culture results were obtained via bag, indwelling catheter, or if multiple organisms or non-pathogenic organisms grew. At this institution, positive dipsticks automatically reflex to a microscopic urinalysis. Primary outcome was a positive urine culture result. UTI’s were defined by colony growth of 50K cfu/ml or greater with a single urinary pathogen on catheterized culture. On clean-catch, UTI’s were defined by 50K cfu/ml or greater with a single urinary pathogen for males and 100k cfu/ml or greater with a single urinary pathogen for females. Pyuria and Leukocyte esterase accuracy were evaluated at 4 different specific gravity groupings.

1.000 to 1.010
1.011 to 1.020
1.021 to 1.030
>1.030

Lowest pyuria cut-off was 5 WBC’s per high-power field.

Outcome

The higher the specific gravity, the lower the positive likelihood ratio and higher the negative likelihood ratio for identification of a true UTI given a particular level of pyuria. Sensitivity of leukocyte esterase decreased with increasing concentration but the same was not noted for pyuria. The Odds ratio was 6.9 (95% Cl 4.1 to 11.5) for positive urine cultures in specimens with a specific gravity less than or equal to 1.030 compared to greater than 1.030.

Definitely read the full study, as it includes a breakdown of the relevant test characteristics across different age breakdowns.

Weaknesses

As the study was evaluating the accuracy of pyuria and leukocyte esterase, these parameters were not included in the definition of UTI (only culture results were). This is appropriate given the study design and is a common limitation of studies evaluating the accuracy of urinary analysis in predicting urinary tract infection. However, this does differ from the AAP guideline definition of urinary tract infection and thus cases of asymptomatic bacteriuria may have been included.

The Bottom Line

Heterogeneity in the predictive ability of urinalysis for detecting UTI may in fact be related to the specific gravity (urinary concentration) of the sample. Pyuria in more dilute urine may be more indicative of an underlying UTI.