Infection Risk in Patients with Mixed Flora in Urine Cultures Prior to Ureteroscopy

Purpose: Urine cultures are routinely used to inform preoperative antibiotic choice and duration prior to endourologic surgery. The presence of mixed flora in preoperative urine cultures holds unclear clinical significance. This study examines infectious outcomes after ureteroscopy in patients with preoperative mixed flora urine cultures. Materials and Methods: A retrospective cohort study was conducted on adult patients who underwent ureteroscopy with laser lithotripsy between January 2014 and June 2024 who had urine cultures performed within 60 days preoperatively. Patients were categorized into cohorts based on their preoperative urine culture: mixed flora, negative, or positive. Postoperative urinary tract infection rates within 30 days were compared between cohorts, and logistic regression was performed adjusting for demographic and clinical variables. Results: We identified 5166 patients who underwent ureteroscopy with laser lithotripsy (2139 mixed flora, 1525 negative, 1502 positive). Preoperative antibiotics were used more often in the mixed flora cohort (29%) than in the negative cohort (24%, p = 0.007) but less frequently than in the positive cohort (57%, p < 0.001). Postoperative infections were visualized in 165 patients (8%) in the mixed flora cohort, compared with 88 (6%) in the negative cohort (p = 0.067) and 237 (16%) in the positive cohort (p < 0.001). Multivariable logistic regression demonstrated that positive cultures were associated with an increased risk of infection (odds ratio [OR] = 1.95, 95% confidence interval [CI] = 1.49-2.55, p < 0.001), but negative cultures had a similar risk of infection compared with mixed flora (OR = 0.79, 95% CI = 0.56-1.11, p = 0.177). Within the mixed flora cohort, preoperative antibiotic treatment was not associated with decreased postoperative infection (OR = 0.99, 95% CI = 0.66-1.47, p = 0.964). Conclusions: While patients with preoperative mixed flora urine cultures received preoperative antibiotics more often than patients with negative urine cultures, they were not at higher risk for postoperative infection. Routine preoperative antibiotic use in patients with mixed flora cultures may not be effective in reducing infectious complications after ureteroscopy.