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-positive cultures, would give similar results in newborns (females and circumcised and uncircumcised males).

DESIGN

Prospective study in which periurethral and urine specimens were obtained from healthy newborns. After the periurethral specimen was obtained, the perineum was washed and a urine bag applied. The urine bag was removed immediately after voiding and the urine was cultured.

SETTING

Normal newborn nursery and pediatric hospital.

SUBJECTS

Ninety-eight healthy full-term newborns (49 female and 49 male) admitted to the normal nursery during a 4-month period.

MAIN RESULTS

Isolation of a pathogen from the bag urine reflected periurethral flora. In 20 (95%) of the 21 urine specimens from which a pathogen was isolated, the same pathogen was detected on the 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 culture was also negative. In the remaining 25 urine samples in which nonpathogens were detected, the periurethra yielded nonpathogens or no growth. 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.

CONCLUSION

This study explains the finding of false-positive cultures with the bag technique. Pathogens detected in bag urine samples reflected pathogens on the periurethra. Until a bag collection technique that avoids contamination by periurethral flora can be developed, urethral catheterization and suprapubic aspiration remain the methods of choice for obtaining a urine specimen in female and uncircumcised male neonates.