Performance of flow cytometry to screen urine for bacteria and white blood cells prior to urine culture

Background: The gold standard test for the diagnosis of urinary tract infection is bacterial culture. However, urine cultures are labor intensive and costly. Furthermore, since results take 1-2. days many patients are treated presumptively prior to culture results being known. Methods: We evaluated the Sysmex UF-1000i for the quantification of bacteria and white blood cells (WBCs) in urine in order to determine if it could be used to predict positive culture in comparison to the use of gram stain as a screening tool. Results: The UF-1000i demonstrated good linearity, within and between run precision for bacterial and WBC quantification. Using ROC analysis, the AUC for predicting a positive culture (>10⁵cfu/mL) was 0.95 and 0.90 for bacteria and WBCs, respectively, with optimum cutoffs of 288.9bacteria/μL and 31.8WBCs/μL, respectively. At these cutoffs, sensitivity (SE) and specificity (SP) for culture positivity were 0.93 and 0.86, respectively, for bacterial counts and 0.89 and 0.79, respectively, for WBC counts. The use of gender specific bacterial cutoffs improved performance, especially in males. In comparison, SE and SP of urine Gram stain were 0.94 and 0.68, respectively. Conclusions: Quantification of bacteria in unspun urine samples by the Sysmex UF-1000i can be used to screen urine samples for those likely to grow >10⁵cfu/mL. The Sysmex UF-1000i demonstrated increased SP over urine Gram stain, and in this study population could reduce unnecessary reflex urine cultures by 55%.