Explanation for false-positive urine cultures obtained by bag technique
T. A. Schlager, J. O. Hendley, S. M. Dudley, G. F. Hayden and J. A. Lohr
Department of Pediatrics, University of Virginia, Charlottesville.
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.
