Urine Specific Gravity and the Accuracy of Urinalysis.

Abstract

A recent study in young infants found that different cutoffs maximized the accuracy of the urine white blood cell count in dilute versus concentrated urine samples. We aimed to confirm this finding and to determine its impact on clinical care. We conducted a retrospective analysis of data gathered on consecutive children <24 months of age with visits to the emergency department during a 5-year period. We evaluated the accuracy of screening tests for urinary tract infection (UTI) in dilute and concentrated urine samples. We also calculated the number of children who would have been treated differently in a hypothetical cohort of 1000 children presenting with fever had urine specific gravity (SG) been taken into consideration. We included 10 078 children. The ability to rule in UTI (as measured by the positive likelihood ratio [LR]) was similar in dilute and concentrated urine for the leukocyte esterase test (11.76 vs 10.71, respectively). The positive LR for urine white blood cell count per high-powered field was higher in dilute urine (9.83 vs 6.12). In contrast, the positive LR for the nitrite test was lower in dilute urine (20.54 vs 47.44). Despite these differences, we found little change in the number of children treated with antibiotics in predictive models that took urine SG into consideration. Although we found that urine SG influences the accuracy of some components of the urinalysis, its inclusion in the decision-making process had negligible effect on the clinical care of children with UTI.