This is the first prospective study that compares the efficacy of automated urine flow cytometry system (UF1000i, UF5000), gram stain and urine cultures in urine specimens of women with uncomplicated urinary tract infections. The results showed that the UF5000 kept a good sensitivity (80.0%) in identifying gram negative bacteria with an acceptable specificity (88.2%). With regard to gram positive bacteria, the UF5000 outperformed the UF1000i in detecting gram positive cocci (UF5000 sensitivity: 70% and specificity: 86.5%) with good specificity which was comparable to gram staining (sensitivity: 60% and specificity: 100%). However, the sensitivity of the UF5000 for identifying mixed growth bacteria was poor.
The UF-5000 is an automated urine analyzer produced by Sysmex Corporation, which performs flow cytometry analysis with a higher level of accuracy and more precise data . Several previous studies investigated the legacy models of automated urine particle analyzers (including the UF-500i and the UF-1000i) for screening urine cultures, while few studies have evaluated the ability of the newer UF5000 in differentiation of bacteria growth patterns. Compared with the legacy systems, the current study showed that the UF5000 kept the comparable sensitivity and specificity for GNB (80.0% and 88.2%, respectively). For specific bacteria (Table 2), the current results revealed that the UF5000 will identify Klebsiella spp. Proteus spp., and Citrobacter spp. while only identifying 37 out of 49 E coli. However, the retrospective study by Kim et al showed good performance of the UF5000 in identifying E coli. Further studies are required to check the performance of the UF5000 in identifying E coli. As for Gram-positive bacteria, the UF 5000 showed high sensitivity and specificity for Enterococcus spp. However, the sensitivity for Streptococci spp. was much lower. In our study, UF 5000 identified all staphylococci spp.(n = 3), enterococci spp. (n = 1), streptococci spp.(n = 3), except one Group B Streptococci (2*104 cfu/mL) and two gram (+) cocci (103 and 2 × 103 cfu/mL, respectively). With regard to gram positive bacteria, the UF5000 outperformed the UF1000i in detecting gram positive cocci which was comparable to gram staining.
Gram staining is associated with a sensitivity rate of 88%, a specificity rate of 95%, a negative predictive value of 96%, and a positive predictive value of 84% for identifying bacteriuria.[6, 14] For differentiating bacterial growth patterns, gram stain yielded good sensitivity and specificity for gram negative (80.6% and 96.7%, respectively) and gram positive bacteria (60% and 100%, respectively). Although a real-time reporting of gram stain could reduce the blind initiation of antibiotics, and thus prevent unnecessary expenditure and drug treatment, gram staining is time consuming and labor-intensive. The UF-5000 offers comparable efficacy and a faster and far easier way to provide the same information as compared to the classic method of Gram staining.
Detecting Gram positive bacteria has significant clinical implications. The most commonly isolated Gram-positive uropathogens are Staphylococcus saprophyticus, Enterococcus faecalis, and Streptococcus agalactiae. One review published in the literature suggests that urologic diseases involving Gram-positive bacteria may be easily overlooked due to limited culture-based assays typically utilized for urine in hospital microbiology laboratories. However, Hooton et al found that only Staphylococcus saprophyticus correlated well with the catheterized urine. However, enterococci and streptococci correlated with catheterized urine culture poorly. Therefore, patients with gram positive bacteria shown on the UF5000 may not have uUTI by classical GNB and may not need empirical antibiotics. Second, in patients with Gram positive cocci determined by UF5000, urine culture is recommended and antibiotics targeting Gram positive bacteria, instead of empirical ones for gram negative bacteria. In this way, we may avoid unnecessary waiting times, overuse of antibiotics and increased medical costs. Third, immediately identifying Gram positive bacteria in ascites, cerebral spinal fluids and pleural fluids may help clinicians make appropriate and timely antibiotic choice in these life threatening infections. More clinical studies to explore the use of the UF-5000 in the situations is encouraged.
About 21.6% of urine cultures revealed mixed growth (n = 22, 21.6%) and lactobacillus (n = 2, 1.9%) which were regarded as contamination due to improper collection, transportation, preservation, and storage. As the study included female participants only, and this may explain the relatively higher contamination rate. Because the short urethral length and the urethra meatus is proximal to the vagina and anus, urine specimen from women were more easily contaminated than men. Our study shows that the sensitivity and specificity of the UF5000 for mixed growth was 4.5% and 94.9%, respectively. Further improvement of laser flowcytometry in identification of mixed growth could help health care and avoid the waste of time, labor and money.
There existed some limitations in our study. Although the study is a prospective study that compared the efficacy of the three methods (gram stain, the UF1000i and the UF 5000i), the major limitation is that the number of samples was limited. Second, a significant proportion of patient specimens yielded gram negative bacteria and mixed growth culture, so the evidence supporting the promising efficacy of the UF5000 in detecting gram positive bacteria is limited due to the low number of specimens with gram positive bacteria. The strength of the study is its prospective nature of the study that compares two automated urine particle analyzers. Further studies are still warranted to evaluate the generalizability of the UF5000 in a larger subset of patients, institutions and populations. Also, the role of the UF5000 in detecting bacterial patterns in the other type of specimens, i.e. ascites, cerebral spinal fluids and pleural effusion, should be investigated further.