Prevalence and predictors of urine culture contamination in primary care: A cross-sectional study

Abstract

Background: Antimicrobial resistance is a global health threat. To slow resistance and preserve antibiotics, stewardship interventions are increasingly promoted and mandated. Urine cultures are the most common microbiological test in the outpatient setting. Contamination most likely occurs during urine collection from surrounding vaginal, perineal, and epidermal flora. Sample contamination can lead to incorrect diagnosis, unnecessary or inappropriate treatment, poor patient outcomes, and higher costs. Therefore, ensuring proper collection of urinary samples serves as a prime diagnostic stewardship target, one that international nursing societies increasingly endorse as an opportunity for nurse involvement.

Objectives: Determine the prevalence, predictors, and antibiotic prescribing associated with contaminated urine cultures in primary care clinics.

Design: Cross-sectional study.

Setting: Two adult safety-net clinics in Houston, Texas.

Participants: 1265 clinical encounters among 1114 primary care patients.

Methods: We reviewed charts from office visits among patients who had a urine culture ordered between November 2018 and March 2020. Patient demographics, culture results and prescription orders were captured for each visit. Culture results were defined as no growth, contaminated (i.e., mixed flora, non-uropathogens, or ≥3 bacterial species isolated), or low-count (10²-10⁵ colony forming units (CFU)/mL) or high-count (>10⁵ CFU/mL) uropathogen-positive. We performed multinomial logistic regression to identify predictors independently associated with contaminated cultures.

Results: Our study evaluated 1265 cultures from 1114 patients that were primarily female (84 %), of Hispanic/Latino (74.4 %) or Black/African American (18.9 %) race/ethnicity with a mean age of 43 years. Out of 1265 urine cultures, 264 (20.9 %) had no growth, 694 (54.9 %) were contaminated, 159 (12.6 %) were low-count positive, and 148 (11.7 %) were high-count positive. Female sex, pregnancy, and obesity were associated with contaminated cultures (multinomial adjusted odds ratios: 15.89, 14.34, 1.93, respectively; 95 % confidence intervals: 10.25-24.61, 8.03-25.61, 1.32-2.81, respectively). Antibiotic prescribing was significantly higher among symptomatic patients with contaminated cultures compared to those with no growth.

Conclusion: Urine culture contamination occurred frequently in our clinics, and obesity, female sex and pregnancy were independent risk factors for contamination. The association of pregnancy and contamination is particularly concerning as pregnant females are routinely screened and treated for asymptomatic bacteriuria in the United States. Culture contamination may obscure underlying uropathogens, leading to pyelonephritis or potential neonatal infection if untreated. Conversely, overtreatment of false positive bacteriuria could lead to adverse effects from antibiotics and increased risk for antibiotic resistance. As nurses play a prominent role in patient education, diagnostic stewardship interventions may want to utilize nurses' educational capabilities to improve urine culture collection.

Tweetable abstract: 55 % of urine cultures collected in primary care clinics were contaminated, revealing a major opportunity for nurse-driven diagnostic stewardship interventions.

Keywords: Antimicrobial stewardship; Bacteriuria; Microbial contamination; Outpatients; Primary health care; Urinary tract infections; Urine specimen collection.