EXPLANATION FOR FALSE-POSITIVE URINE CULTURES OBTAINED BY BAG TECHNIQUE

Objective: To test whether a urine bag technique, previously shown in circumcised male infants 1 month to 1 year of age to yield no false-po sitive cultures, would give similar results in newborns (females and c ircumcised and uncircumcised males). Design: Prospective study in whic h periurethral and urine specimens were obtained from healthy newborns . After the periurethral specimen was obtained, the perineum was washe d and a urine bag applied. The urine bag was removed immediately after voiding and the urine was cultured. Setting: Normal newborn nursery a nd pediatric-hospital. Subjects: Ninety-eight healthy full-term newbor ns (49 female and 49 male) admitted to the normal nursery during a 4-m onth period. Main Results: Isolation of a pathogen from the bag urine reflected periurethral flora. In 20 (95%) of the 21 urine specimens fr om which a pathogen was isolated, the same pathogen was detected on th e periurethra. Sixteen of the 21 urine cultures were falsely positive (>10(4) colony-forming units of pathogen per milliliter). In 50 (98%) of the 52 urine samples that yielded no growth, the periurethral cultu re was also negative. In the remaining 25 urine samples in which nonpa thogens were detected, the periurethra yielded nonpathogens or no grow th. Thus, if a pathogen was isolated from a bag urine sample, the same pathogen was detected on the periurethra 95% of the time. Conversely, if the bag urine sample was negative for a pathogen, the periurethral culture was negative 100% of the time. The presence of a pathogen on the periurethra was more common in female than male neonates (16 of 49 vs four of 49; P=.004), and none of the 14 circumcised male neonates had a pathogen detected on their periurethra or in their urine. Conclu sion: This study explains the finding of false-positive cultures with the bag technique. Pathogens detected in bag urine samples reflected p athogens on the periurethra. Until a bag collection technique that avo ids contamination by periurethral. flora can be developed, urethral ca theterization and suprapubic aspiration remain the methods of choice f or obtaining a urine specimen in female and uncircumcised male neonate s.