Rapid detection of urinary tract infection – evaluation of flow cytometry

Abstract

Urogenital tract infection (UTI) due to bacteria is not only a common infection but also a complication during hospitalization. Therefore, the identification and quantification of bacteria in urine samples are routinely performed methods in microbiological laboratories. To differentiate between infection and contamination it is also important to quantify the leukocyte count. In this study, we suggest a screening procedure using the flow cytometer analyzer BACSYS-40i as tool for the diagnosis of UTI. Each urine sample was inoculated onto agar plates (MacConkey agar, sheep blood agar and enterococcosel agar (Bio Merieux, Nütingen, Germany)) within 4 hours after collection. After 24 up to 48 hours incubation at 37 degrees C, bacteria were quantified by evaluation of colony-forming units (CFU) according to the criteria given by the German recommendation [MIQ 1997]. Additionally, each sample was submitted to the BACSYS-40i analyzer. The technological principle is a fluorescence flow cytometer with a laser and a fluorescent dye to identify bacteria and leukocytes with high analytical sensitivity. Coefficients of variation (CV) for examination of within-run reproducibility ranged from 1.7 - 9.0% for leukocytes and from 6.2 - 24.6% for bacteria. Linearity was found to be very good, with coefficients of determination of r = 0.9998 for leukocytes, and r = 0.9994 for bacteria. Carry-over was calculated and found to be extremely low, ranging up to 0.03% for leukocytes and up to 0.002% for bacteria. The correlation coefficient for leukocyte counting is 0.979, regression y = 1.0 x + 1.0. The number of bacteria determined with the BACSYS-40i (total cell count) is higher than the number determined by culture (viable cell count). If the cut-offs of the analyzer were fixed at > or = 10(6) and < 10(5) bacteria/microl for positive, respectively negative results, 39 out of 42 patients (93%) showing unambiguous predominant clinical signs of UTI and in addition growth of bacteria involved in UTI were recognized. Six samples were questionable. Results obtained by the BACSYS-40i can be reported after a few minutes. Urine samples from all 57 patients with predominant clinical signs and in addition growth of bacteria isolated from urine and known as pathogens of UTI had positive results with the analyzer (100%) for elevated bacteria and leukocyte counts. Furthermore, all patients without symptoms for UTI were negative after analysis with the BACSYS-40i (44/44; 100%).