Rate of Urine Culture Contamination with Different Methods of Urine Specimen Collection

Midstream urine (MSU) samples are commonly collected at the time of patient evaluation despite known high rates of contamination.

The primary objective of this study was to evaluate the rate of mixed flora results in urine specimens obtained by MSU compared to straight catheterization urine (SCU).

This was a quality improvement project evaluating urine culture results of women who provided either an MSU or SCU sample for analysis. Adult women seen within urogynecology clinics at a tertiary care center between April and August 2023 who had urine cultures performed for any indication were included. Mixed flora was defined as the presence of ≥ 2 non-uropathogens or 1 uropathogen in low quantity (at least 10 times fewer) compared to the concentration of nonsignificant organisms.

Three hundred forty women provided a urine specimen during the study period. SCU collection was performed for 171 (50.3%) women while 169 (49.7%) provided an MSU sample. Overall, 18.8% of urine cultures were reported as mixed flora (33.1% in MSU and 4.7% in SCU, p < 0.001). Mixed flora was more common with MSU specimens (87.5%, p < 0.001) and associated with a higher BMI compared to positive or negative cultures (mixed flora 29.8 kg/m² ± 16.3, positive or negative cultures 27.8 kg/m² ± 7.0, p = 0.04). MSU samples had increased odds of urine contamination compared to SCU collection (7.40 aOR, 95% CI 3.01-18.24).

The prevalence of mixed flora was reduced significantly when SCU samples were obtained. Clinicians should consider performing SCU collection when a urine specimen is required for patient evaluation.