Investigating urinary characteristics and optimal urine white blood cell threshold in paediatric urinary tract...

While the definitive diagnosis of urinary tract infection (UTI) requires a positive urine culture, the likelihood of UTI can be determined by urinalysis that includes white blood cell (WBC) count. We aimed to determine the optimal urine WBC threshold in urinalysis to predict UTIs in children presenting at the emergency department (ED).

We performed a prospective observational study in the ED at KK Women's and Children's Hospital for children below 18 years old who underwent both urine microscopy and urine cultures, between 10 January and 7 November 2022. We assessed the various urine WBC thresholds associated with culture-proven UTIs using sensitivity, specificity, positive and negative predictive values, positive and negative likelihood ratios, and area under receiver operating characteristic curve.

We found a culture-proven UTI rate of 460/1188 (38.7%) among all patients analysed, and 278/998 (27.9%) among those with nitrite-negative urine samples. Among all patients, a urinalysis WBC threshold of 100/μL had a sensitivity of 82.2% (95% confidence interval [CI] 78.4-85.5) and negative predictive value of 86.2% (95% CI 83.6-88.4). Among those who were nitrite-negative, a WBC threshold of ≥100/μL resulted in a potential missed rate of 48/278 (17.3%). By lowering the WBC threshold to ≥10/μL, the potential missed cases reduced to 6/278 (2.2%), with an estimated increase in 419 urine cultures annually.

A urine microscopy WBC threshold of ≥100/μL results in a clinically significant number of missed UTIs. Implementation of various thresholds should consider both the potential missed UTI rate and the required resource utilisation.