Clinical and microbiological investigation into mixed growth urine cultures

Introduction. Urine samples submitted for investigation of urinary tract infection (UTI) may identify more than one bacterial isolate. These samples may be reported as ‘mixed growth urine culture’ (MGUC). The clinical significance of MGUC remains controversial.

Hypothesis/Gap Statement. The impact of MGUC on patient management is not known and should be assessed.

To describe MGUC and assess its impact on patient management.

Methodology. Microbiology laboratory reports (Leeds, UK) were retrospectively analysed and urine cultures reported as MGUC from a 1 week period underwent detailed laboratory analysis. Semi-structured interviews of NHS clinicians’ response to MGUC reports were explored for emergent themes.

Results. In 2018, 12.4 % (14,323/115,664) of urine specimens processed to detect bacterial pathogens were reported as MGUC. Among a total of 200 MGUC samples identified within 1 week in 2019, detailed laboratory analysis identified 459 bacterial isolates. Enterococcus species (30.1 %) and Escherichia coli (27.5 %) were the most frequently isolated and the most frequent organism combination (24 %). In total, 65.5 % cultures contained two organisms and 82.5 % of all MGUC contained at least one Enterobacterales. Interviews found clinicians believed MGUC reports represented detection of many commensal bacteria. Clinicians indicated they were more likely to diagnose and treat a UTI when provided with urine culture reports derived from detailed microbial analysis of MGUC, including identity and antibiotic sensitivity of organisms.

Conclusions. This study highlights the potential underuse of information derived from microbiological analysis of urine samples. Interpretive commentary on reports together with education for interpretation of enhanced reports should be explored further to improve outcomes in patients with UTI.