Mixing It Up: Are Patients With Mixed Urine Cultures at Higher Risk of Postoperative Sepsis After Ureteroscopy?

A retrospective multi-institutional chart review was performed on 313 patients undergoing elective ureteroscopy with laser lithotripsy were divided based on preoperative cultures into the “negative”, “mixed”, or “positive” urine culture group.  The mixed culture group was composed of patients with culture reports of >10,000 colony forming units (CFU) mixed urogenital flora, mixed gram positive and gram negative, and Lactobacillus, as these were generally consigned benign or contaminated. Positive cultures were defined as >100,000 CFU of single or multiple organisms. Patient charts were analyzed for factors including stone composition, stone size, stone weight, preoperative urine culture, preoperative antibiotic, stent indwelling time prior to procedure, and whether patients were admitted postoperatively or readmitted within 48 hours after procedure for suspicion of sepsis. Sepsis was defined as two systemic inflammatory response syndrome (SIRS) criteria (temperature >100.4F or < 96.8F, heart rate >90 beats per minute, respiratory rate >20 breaths per minute, and white blood cells count >12,000 cells/mm3), and a source of urinary infection.  A one tailed Fisher's exact test was chosen to determine if one group had significantly higher postoperative sepsis rates than the other. Mann Whitney U Test/Wilcoxon rank-sum test was used for continuous dependent variables and categorical independent variables where the data does not follow a normal distribution. Logistic fit was used for categorical dependent variables and continuous independent variables. Data were reported as the Mean ± Standard Deviation for numerical variables, n (%) for categorical variables and adjusted ORs (AOR; 95% confidence intervals [CI]). Statistical significance was accepted at P < .05.




Outcomes:

The overall sepsis rates were 3.6% with negative culture preoperatively, 10.8% with a mixed culture, and 20.5% with positive culture (Figure 1). When comparing the mixed, negative, and positive culture groups, a statistically significant difference was observed between the mixed and the negative group (p=0.03). There was no statistically significant difference between mixed and positive cultures (p=0.27) (Table 1). There was also a statistically significant sepsis rate between the positive and negative culture groups (p=0.0012). Stone weight, size, and composition were also compared between mixed, negative, and positive culture groups. Stent duration was shown to not be associated in  the mixed culture group compared to negative culture (p=0.54). The positive group also had greater stent duration compared to the negative group (p=0.03). There were no differences between the mixed and positive groups. Stone weight was significantly greater in mixed culture groups compared to negative culture groups (p=0.04). No differences were found between positive culture and negative cultures or mixed culture and positive cultures. There was a significant difference in oxalate monohydrate between negative and mixed cultures (p=0.01). A difference was also shown in carbonate apatite between mixed culture group and negative culture group (p=0.01) and positive culture group and negative culture group (p=0.02) (Table 1).
There was an increased duration in the sepsis group compared to the non sepsis group with mean duration 34.1 days compared to 12.9 days (p < 0.002). There were differences in the oxalate dihydrate group with mean composition of 17.1% in the sepsis group compared to 32.0% in the non sepsis group (p=0.03) and carbonate apatite group with mean composition of 33.5% in the sepsis group compared to 17.4% in the non sepsis group (p=0.01) (Table 2).





Conclusion: We report that preoperative mixed urine cultures were associated with 3.6 times higher rates of sepsis in study patients undergoing ureteroscopy with lithotripsy and stent placement. Both the positive culture and mixed culture groups had greater proportions of carbonate apatite stone composition and longer indwelling stent time, suggesting these factors contribute to the increased sepsis rates. Carbonate apatite stones and encrusted stents could harbor and sequester bacteria, leading to only a small amount of uropathogens growing on culture that could be outcompeted by urogenital flora and thus reported as “mixed”. Subsequent ureteroscopic lithotripsy of these mixed culture patients then leads to release of the uropathogens into the surrounding environment, triggering sepsis. Our findings add to a growing body of literature on the current ambiguity of managing these common “mixed culture, probably contamination” reports and prompt the consideration of new management strategies.