Urine culture contamination: a College of American Pathologists Q-Probes study of contaminated urine cultures in 906 institutions

Abstract

Objective: To examine the frequency and causes of urine culture contamination in outpatients.

Methods: Nine hundred six institutions, ranging in size from less than 50 to more than 600 beds, each examined 250 consecutively ordered urine cultures from outpatients and identified specimens that met the study definition of contamination. Participants also answered questions about the practices used to collect, transport, and process urine specimens. In all, data relating to more than 200,000 urine cultures were analyzed.

Results: The median institution reported that 18.1% of urine cultures specimens collected from outpatients were contaminated. The top 10% of institutions (90th percentile) reported that 5.6% or fewer of their cultures were contaminated. In contrast, the bottom 10% of institutions (10th percentile) reported that 36.8% or more of their urine specimens were contaminated. Institutions with lower contamination rates tended to process a lower proportion of specimens from female patients. Pediatric hospitals also reported lower contamination rates in specimens from children than general hospitals. Other factors, including the use of central processing areas, refrigeration, urine screening systems, specimen preservatives, provision of written collection instructions or special collection kits, and thermally insulated specimen transport containers, were not found to be associated with low specimen contamination rates in a multivariate analysis.

Conclusion: Contamination of outpatient urine cultures is a common occurrence, and facilities differ significantly in their overall frequency of urine culture contamination. Many interventions commonly assumed to reduce contamination were not demonstrably effective in this study.