High False-Positive Rate with Bag Urine Collection in Pediatrics
Urine cultures obtained by bag collection in pediatric patients have an unacceptably high false-positive rate of approximately 44-61%, making this method unreliable for diagnosing urinary tract infections.
The Evidence on False-Positive Rates
The most comprehensive systematic review and meta-analysis from 2016 demonstrates that bag collection (SUB - sterile urine bag) has a specificity ranging from 56.5% to 92.3% when compared to gold-standard methods like suprapubic aspiration (SPA) or catheterization 1. This translates to a false-positive rate of 7.7% to 43.5% depending on the study.
More specifically:
- Aronson et al. found bag collection had a sensitivity of 83.3% but specificity of only 56.5%, meaning 43.5% false-positive rate 1
- Hardy et al. reported bag collection sensitivity of 50.0% with specificity of 92.3%, yielding a 7.7% false-positive rate 1
- Grisaru-Soen et al. (good quality study) showed bag collection had 100% sensitivity but specificity of only 83.7%, resulting in a 16.3% false-positive rate 1
A 2016 systematic review specifically examining bag collection validity found the pooled false-positive rate was 61.1% (95% CI: 37.9-82.2%) across 12 studies with 575 samples 2. This means that when a bag urine culture is positive, approximately 2 out of 3 results are false positives 2.
Contamination Rates Are Equally Problematic
Beyond false positives, contamination rates for bag collection range from 26.6% to 67.6% across multiple studies 1:
- Karacan et al.: 43.9% contamination rate (227/517 specimens) 1
- Alam et al.: 26.6% contamination rate (45/169 specimens) 1
- Macfarlane et al.: 67.6% contamination rate (23/34 specimens) 1
The pooled contamination percentage across 15 studies with 6,856 samples was 46.6% (95% CI: 35.6-57.8%) 1.
Clinical Impact: Misdiagnosis Risk
A prospective study by Etoubleau et al. (2009) directly compared bag versus catheter specimens in the same children and found that bag collection led to either misdiagnosis or impossible diagnosis in 40% of cases versus only 5.7% with catheterization 3. Specifically:
- 7.5% false-positive rate (treating children who don't have UTI)
- 29% false-negative rate (missing true UTIs)
Why Bag Collection Fails
The mechanism of contamination is well-established: pathogens detected in bag urine samples directly reflect periurethral flora contamination 4. In a study of newborns, when a pathogen was isolated from bag urine, the same pathogen was detected on the periurethra 95% of the time 4.
Risk factors for contamination include:
- Uncircumcised males (higher contamination) 5
- Female infants (16/49 vs 4/49 males had periurethral pathogens, P=0.004) 4
- Waiting for more than one void to complete collection 5
Clinical Recommendation
Do not rely on bag urine cultures for diagnosis of UTI in pediatrics. If a bag specimen shows growth:
- Always confirm with catheterization or suprapubic aspiration before initiating antibiotic therapy 3
- Consider the positive result as a screening test only, not diagnostic
- In one quality improvement study, implementing this approach reduced false-positive treatment from 74% to 17% of cases 6
The evidence consistently shows that catheterization or suprapubic aspiration should be the default collection methods when UTI diagnosis is being considered, as these have contamination rates of only 3.5-5.7% compared to bag collection's 26-68% 7.