Optimising the diagnosis of UTI in patients with mixed growth bacteriuria

Background: Leeds Teaching Hospitals NHS Trust (LTHT) Microbiology department report urine cultures with >1 isolate as ‘mixed growth’ (MG). The clinical significance of this remains controversial. We aimed to describe MG and explore how clinicians interpret MG reports. Methods: Urine culture data (2018) were retrospectively collected from the LTHT laboratory information system. Urine cultures reported as MG from a one-week period in 2019 underwent further laboratory analysis. Semi-structured interviews of eight NHS clinicians were explored for emergent themes around MG using framework analysis. Clinicians were also provided with data in existing MG report formats and alternative reports including organism identities and sensitivities; and clinician propensity to diagnose/treat urinary tract infection (UTI) was noted. Results: Of 115664 cultured specimens, 12% were MG with rates highest in patients >65yrs (19.3%, n=41760) and <1yr old (18.7%, n=2727). Of 459 isolates from 200 MG cultures, Enterococcus species (30.1%) and E. coli (27.5%) were most frequently isolated and the most frequent organism combination (24%). 65.5% of cultures contained 2 organisms, with 82.5% having at least one Enterobacteriales. Many clinicians believed MG statements represented detection of many commensal bacteria which did not represent infection. When given alternative reports including organism identities/sensitivities, (cf. report of MG only), more were likely to diagnose and treat a UTI Conclusions: MG in urines is common. Most MGs include Enterobacteriales, an important cause of UTI. Reporting two species as MG only can be misleading, resulting in failure to treat clinically apparent infections.