Clinical significance of mixed bacterial cultures of urine

The frequency with which isolation of more than one bacterial species from urine signifies treatable mixed infection versus contamination or colonization is not known. The authors studied 247 patients yielding mixed urine cultures. Specimens were collected by clean catch (CC) from 88 and from closed drainage systems (CDS) from 159. A second specimen was collected within 48 hours, and the results of the two cultures were compared. The percentages in which the initial mixed culture was found to represent probable, possible, and improbable treatable mixed infection were as follows: for CC specimens, 11%, 20%, and 67%, and for CDS specimens, 3%, 21%, and 77%. The authors have found that empiric antibiotic therapy and reporting of mixed cultures based on culture morphology without complete identification or antibiotic susceptibilities (except for certain colony types suggesting potentially multi-drug resistant strains) with request for resubmission represents a cost-effective solution to the mixed culture problem in the diagnosis and treatment of urinary tract infection.