Rapid direct-from-urine growth detection to triage culture-negative urine: a prospective diagnostic accuracy study of a two-tube nitrate-reduction and bromothymol blue assay (SIBURI/SIBURI+)

Objectives: Urine culture takes 24 to 72 h to report, and most specimens are culture-negative. We evaluated whether a 6-h direct-from-urine growth-detection assay can identify culture-negative specimens.Methods: In a prospective single-centre study at Goa Medical College, 145 consecutive urine specimens from patients with suspected urinary tract infection (UTI) were tested in parallel. Urine (50 uL) was added to SIBURI (nitrate-reduction broth, developed with Griess reagent) and SIBURI+ (Luria-Bertani broth with bromothymol blue) and incubated for 6 h at 37 C. Any visible colour change against an uninoculated blank was scored growth-positive. The reference standard was quantitative culture (greater than or equal to 10^5 CFU/mL of a single organism). Twenty-five specimens reported as contaminated, SCC or with greater than or equal to 3 organisms were excluded. Accuracy was estimated with exact 95 percent confidence intervals. A cell-free nitrite series and ATCC Escherichia coli and Klebsiella pneumoniae dilution series characterised the colour response.Results: Of 120 analysed specimens, 41 (34.2 percent) were culture-positive. Sensitivity was 92.7 percent (95 percent CI 80.1 to 98.5), specificity 92.4 percent (84.2 to 97.2), negative predictive value (NPV) 96.1 percent (88.9 to 99.2) and positive predictive value 86.4 percent (72.6 to 94.8). SIBURI alone detected 29 of 41 culture-positive specimens (70.7 percent); nine (enterococci, streptococcus, Acinetobacter, Candida and two unspecified) were detected only by SIBURI+. Classifying the excluded specimens as culture-positive gave a sensitivity of 93.9 percent and an NPV of 94.8 percent; classifying them as culture-negative lowered specificity to 71.2 percent. The colour response was biphasic: pink to red from 10 uM to 1 mM nitrite, and yellow above 1.5 mM.Conclusions: In this single-centre study, a two-tube 6-h assay showed good accuracy against culture, but the lower confidence limits (sensitivity 80.1 percent, NPV 88.9 percent) do not yet support omitting culture after a negative result. Positive results need culture confirmation, and larger multicentre validation is required.