Quality indicators for urine sample contamination: can squamous epithelial cells and bacteria count be used to identify properly collected samples?

Objectives: To evaluate urinalysis parameters useful for identifying mixed cultures in urine culture using an automated urinary particle analyzer to assess quality indicators (QIs) for urine sample contamination.

Methods: A retrospective observational cross-sectional study was conducted with 2,527 urine samples from patients of a quaternary hospital in Brazil. Urine samples were processed simultaneously in Sysmex UF-5000 flow cytometry analyzer (urinalysis) and MALDI-TOF (culture).

Results: For all samples, a cutoff of 123.8 bacteria/µL was established to discriminate culture-negative specimens. ROC curve indicated the following cutoffs for females and males, respectively: 193.65 and 23.55 bacteria/µL, and 21.35 and 5.05 squamous epithelial cells (SEC)/µL, with the latter two related to scenarios of sample contamination/colonization through mixed cultures. Performing univariate logistic regression, we found a 2.78 (CI95 %: 2.12-3.65) times higher chance of probable mixed culture when SEC values were above the cutoffs for each sex, and 6.91(CI95 %: 4.56-10.47) times for bacteria. For multivariate logistic regression, the OR values were 1.62 (CI95 %: 1.21-2.15) and 5.82 (CI95 %: 3.77-8.98), respectively.

Conclusions: The fluorescent flow cytometry analyzers could efficiently identify urinary bacteria counts associated with contamination/colonization scenarios using the cutoffs of 21.35 SEC/µL for women and 5.05 SEC/µL for men. The cutoffs for bacteria/µL (193.65 for females and 23.55 for males) indicated that the presence of bacteria in male samples may be more associated with urinary tract infections (UTIs), while in female samples, it may be associated with either UTIs or contamination/colonization scenarios. This makes the analyzer a helpful tool as QI of sample contamination in urine cultures.