Choice of Urine Collection Methods for the Diagnosis of Urinary Tract Infection in Young, Febrile Infants  Alan R Schroeder ¹, Thomas B Newman ¹, Stacia A Finch ², Richard C Wasserman ^2,3 and Robert H Pantell ¹. ¹ Pediatrics, UCSF, San Francisco, CA ; ² Pediatric Research in the Office Setting (PROS), Dept of Practice & Research, Ctr for Child Health Research, AAP, Elk Grove Village, IL and ³ Dept of Pediatrics, University of Vermont, Burlington, VT.

Background: To diagnose urinary tract infection (UTI) in febrile infants, urine is most often obtained by sterile bag or urethral catheterization (CATH). CATH is thought to produce fewer false positive culture results, but is more invasive and technically difficult.

Objective: To determine the predictors of urine collection method and to compare culture results of specimens obtained by bag and CATH.

Design/Methods: We used data from the Pediatric Research in the Office Setting (PROS) Febrile Infant Study, an observational study of 3066 infants aged 0-3 months with temperatures 38 C. Infants were tested and treated at the discretion of practitioners. Odds ratios (OR) for predictors of CATH were calculated using a multivariate logistic regression model, clustering by physician. We compared rates of ambiguous cultures, UTI with bacteremia, and UTI across methods. UTI was defined as pure growth of a pathogenic organism with 100,000 cfu/mL for bag cultures and 20,000 cfu/mL for CATH cultures.  Ambiguous cultures were defined as either (a) meeting the colony count definition for UTI but also having a second organism or (b) having 20-99% of the colony count threshold (pure growth).

Results: Urine cultures were obtained on presentation in 1605 infants (52%); 69% by CATH and 24% by bag. Significant predictors of catheterization included: female sex (OR = 2.0, p<.001), Medicaid (OR = 1.7, p<.001), and practitioner age <40 years (OR = 2.8, p=.001 compared with >49 years). Crude UTI rates were similar in bag and CATH (8.5% vs 10.8%; adjusted OR for UTI in CATH = 1.3, p=.32), and rates of UTI with bacteremia were the same (1%). Ambiguous culture results were more common in bag than CATH (7.4% vs 2.7%, p<.001). However, 21 cultures would have to be obtained by CATH to avoid one ambiguous culture.

Conclusions: Severity of illness is less predictive of catheterization than fixed variables such as patient sex, practitioner age, and Medicaid insurance. The adjusted UTI rate was not significantly different between bag and CATH methods, suggesting that false positive results are not more common in bag cultures, or are partially balanced by false negatives. Ambiguous culture results are more common in bag cultures, but a large number of catheterizations need to be done to prevent one ambiguous culture.