Clinical question: In infants younger than three months of age with bacteremic urinary tract infection (UTI), how sensitive and specific are urinalysis (UA) findings?
Background: Infants are commonly hospitalized with UTIs. The gold standard for diagnosis is considered to be urine culture. When compared to this gold standard, the sensitivity of UA findings for the diagnosis of UTI has been previously reported to be around 75% to 85%; however, a positive urine culture alone in the setting of negative UA may not be reflective of a UTI due to asymptomatic bacteriuria or contamination. The 2011 American Academy of Pediatrics clinical guideline for UTIs suggests that the diagnosis should require positive urine culture in addition to abnormal UA. These guidelines do not include infants younger than two months of age, and positive cultures in this age group are generally regarded as a UTI and treated as such. Positive culture results with the same organism in the urine and blood indicates very low likelihood of contamination or asymptomatic bacteriuria, and patients with bacteremic UTI are likely to have a true infection.
Study design: Multicenter, retrospective, cross-sectional study.
Setting: Twenty hospitals in eleven hospital systems.
Synopsis: Researchers used a multicenter microbiology database to identify infants younger than three months of age with bacteremic UTI (same pathogenic organism in blood and urine). Data was collected on UA, including microscopy [white blood cells per high-power field (WBC/HPF), bacteria], dipstick [nitrites, leukocyte esterase (LE)], and urine culture in colony-forming units per mL (CFU/mL).
- Major comorbidities (defined in this study as neuromuscular conditions such as spina bifida, previous urologic surgery other than circumcision, or immunodeficiency);
- Patients managed in an ICU setting; and
- Patients with indwelling urinary or central venous catheters at the time of culture.
A total of 276 infants with bacteremic UTI were identified, with 31 exclusions (12 with no UA performed, 19 with cultures with <50,000 CFU/mL). The remaining 245 infants were included for analysis. The control group was a random sampling of 115 similarly aged infants who underwent evaluation for serious bacterial infection and had negative urine cultures.
Comparison between the study group (bacteremic UTI) and the controls showed:
- LE (including any “positive” LE) had a sensitivity of 97.6%, specificity of 93.9%;
- Considering “trace” LE as negative changed the sensitivity and specificity to 95.7% and 97.4%, respectively; and
- Positive nitrites had a specificity of 100%.
A definition of positive UA that includes pyuria (greater than 3 WBC/HPF) and/or any LE was highly sensitive (99.5%) and specific (87.8%). All but one of 203 infants with bacteremic UTI who had complete UA results were positive for LE and/or WBC/HPF. The one exception was a 64-day-old girl with Group B Streptococcus infection. Bacteria on microscopy showed poor specificity.
The authors discussed two possible explanations for the study’s finding of high sensitivity of the UA, including:
- The UA is in fact highly sensitive, and previous studies have been flawed by a faulty gold standard (positive cultures due to asymptomatic bacteriuria or contamination); or
- Screening tests are more sensitive in the setting of severe disease (in this case, UTI with bacteremia).
The second explanation is controversial, and the authors of this article cite previous studies showing minimal differences between UTI with or without bacteremia.
Bottom line: In infants younger than three months of age with bacteremic UTI, the findings of pyuria and/or any LE on UA are reliable predictors of infection, with higher sensitivity than previously reported.
Citation: Schroeder AR, Chang PW, Shen MW, Biondi EA, Greenhow TL. Diagnostic accuracy of the urinalysis for urinary tract infection in infants <3 months of age. Pediatrics. 2015;135(6):965-71.