Screening for urinary tract infection with the Sysmex UF-1000i urine flow cytometer

Abstract

The diagnosis of urinary tract infection (UTI) by urine culture is time-consuming and can produce up to 60 to 80% negative results. Fast screening methods that can reduce the necessity for urine cultures will have a large impact on overall turnaround time and laboratory economics. We have evaluated the detection of bacteria and leukocytes by a new urine analyzer, the UF-1000i, to identify negative urine samples that can be excluded from urine culture. In total, 1,577 urine samples were analyzed and compared to urine culture. Urine culture showed growth of ≥10(3) CFU/ml in 939 samples (60%). Receiver operating characteristics (ROC) curves and ROC decision plots were been prepared at three different gold standard definitions of a negative urine culture: no growth, growth of bacteria at <10(4) CFU/ml, and growth of bacteria at <10(5) CFU/ml. Also, the reduction in urine cultures and the percentage of false negatives were calculated. At the most stringent gold standard definition of no growth, a chosen sensitivity of 95% resulted in a cutoff value of 26 bacteria/μl, a specificity of 43% and a reduction in urine cultures of only 20%, of which 14% were false negatives. However, at a gold standard definition of <10(5) CFU/ml and a sensitivity of 95%, the UF-1000i cutoff value was 230 bacteria/μl, the specificity was 80%, and the reduction in urine cultures was 52%, of which 0.3% were false negatives. The applicability of the UF-1000i to screen for negative urine samples strongly depends on population characteristics and the definition of a negative urine culture. In our setting, however, the low workload savings and the high percentage of false-negative results do not warrant the UF-1000i to be used as a screening analyzer.

Fig. 1.

ROC curves for bacterial detection (blue) and leukocyte detection (green) by the UF-1000i when no growth (left), <10⁴ CFU/ml (middle), and <10⁵ CFU/ml (right) were chosen as the gold standard definitions for negative cultures.

Fig. 2.

ROC decision plots for the detection of bacteria by the UF-1000i, depicting sensitivity (purple) and specificity (cyan) as a function of the cutoff value chosen for the UF-1000i when no growth (left), <10⁴ CFU/ml (middle), and <10⁵ CFU/ml (right) were chosen as gold standard definitions for negative cultures.

Fig. 3.

The percentage of negative UF-1000i test results at different cutoff values (dark blue) and the percentage of false-negative UF-1000i test results among the total UF-1000i negative test results (red) when no growth (left), <10⁴ CFU/ml (middle), and <10⁵ CFU/ml (right) were chosen as gold standard definitions for negative cultures.