Efficacy of Automated Urinalysis in Detecting Low-Level Bacteriuria and Discriminating Bacterial Gram Status

BACKGROUND: Rapid and accurate diagnosis of urinary tract infections (UTIs) is essential to guide empirical therapy and curb antimicrobial resistance. However, conventional urine culture is too slow, hindering timely diagnosis of UTI. UF‑5000, an automated flow cytometry urinalysis system equipped with the BACT-Info Gram-prediction feature, shows potential to expedite microbial screening. However, its performance under low bacterial loads re-mains unclear. METHODS: We retrospectively analyzed 1,529 clinical urine specimens using urinalysis and urine culture. Any bacterial growth in urine culture was considered a culture-positive result. Receiver operating characteristic (ROC) analysis of UF‑5000 bacterial and white blood cell counts was performed to determine the optimal cutoffs. The agreement between the BACT‑Info flag feature and urine culture results was assessed using Cohen's κ value. RESULTS: Urine cultures identified bacteria in 700/1,529 specimens (45.8%), with 585 (38.3%) showing ≥ 105 colony-forming units/mL. ROC analysis established a bacterial count cutoff of 195.2/µL, yielding a sensitivity of 0.851, specificity of 0.885, and area under the ROC curve of 0.940 (95% confidence interval, 0.928 - 0.951). It outper-formed the cutoff value based on white blood cell count. At equivalent amounts of bacterial culture, UF‑5000 bacterial counts were significantly higher for Gram‑negative than for Gram‑positive isolates (Welch's t-test: p < 0.001). Overall concordance between BACT‑Info and urine culture results was substantial for Gram‑negative flags (κ, 0.76; accuracy, 0.904) and moderate for Gram‑positive flags (κ, 0.51; accuracy, 0.799). For Gram-negative bacteria, the sensitivity, specificity, positive predictive value, and negative predictive value were 0.739, 0.974, 0.923, and 0.898, respectively. However, 17.2% of the specimens containing only Gram-negative bacteria were flagged as "Gram Positive?". This indicates the potential for improvement. CONCLUSIONS: The UF‑5000 system provides a rapid, low-labor, and cost-effective method for UTI screening, even at low levels of bacteriuria. However, it is intended to complement, not replace, conventional urine culture, which remains indispensable for definitive species identification and antimicrobial susceptibility testing. Its primary value lies in accelerating the diagnostic workflow and reducing unnecessary cultures when used in conjunction with urine culture.