To evaluate the test performance of microscopic bacteriuria by automated urinalysis for presumptive urinary tract infection (UTI) in young children.

This is a retrospective cross-sectional study of children aged <2 years evaluated for UTI in a single large emergency department with paired automated microscopic urinalysis and culture. Test characteristics were calculated for automated microscopic bacteriuria and pyuria, and a practical diagnostic threshold of bacteriuria was determined. Standard test performance measures and receiver operator characteristic curves were generated. The diagnostic performance of bacteriuria was compared with microscopic pyuria.

Two thousand five hundred fifty-four children with a median age of 6.1 months were studied, 19% of whom had a positive urine culture. Automated microscopic bacteriuria ≥1+ resulted in a positive likelihood ratio (LR+) of 4.5 (95% CI, 3.9-5.2) and negative LR (LR-) of 0.52 (95% CI, 0.47-0.57). Pyuria alone (≥5 WBC/high-power field) had a LR+ of 4.5 (95% CI, 4.1-5.0) and a LR- of 0.14 (95% CI, 0.11-0.18), whereas the addition of automated microscopic bacteriuria ≥1+ improved the LR+ to 16.3 (95% CI, 12.6-21.1) but raised the LR- to 0.51 (95% CI, 0.47-0.56). Test performance of automated microscopic bacteriuria measured by area under the curve analysis was lower (0.73; 95% CI, 0.70-0.76) than for pyuria (0.92; 95% CI, 0.90-0.93). Isolated automated microscopic bacteriuria without pyuria occurred in only 204 patients (8.0%), among whom only 20 (9.8%) had a positive urine culture.

Microscopic bacteriuria measured by automated urinalysis augments the diagnostic value of pyuria for identifying presumptive UTI in young children aged <2 years. Bacteriuria is diagnostically inferior to microscopic pyuria, and in children with bacteriuria without pyuria, presumptive UTI is unlikely.