Continuous low-concentration hypochlorous acid for microbial control of endoscope final-rinse water pipelines: a multicenter prospective Study in Shanghai, China

Abstract Background Microbial contamination of final-rinse water can recontaminate flexible endoscopes after high-level disinfection. We evaluated whether continuous low-concentration hypochlorous acid (HOCl) treatment was associated with sustained microbiological control of final-rinse water pipelines in hospital endoscopy units. Methods This prospective multicenter before-and-after study included 65 fixed sampling sites (39 automated endoscope reprocessor [AER] outlets and 26 manual-rinse sinks) in 13 tertiary hospitals in Shanghai, China, from May to November 2025. An online HOCl generator was installed at the midpoint of the main final rinse water pipeline to deliver a continuous concentration of 2 mg/L HOCl. Two baseline water samples were collected before installation, followed by repeat sampling on days 1, 7, 30, and 90. Total bacterial count, bacterial identification, endotoxin, and conductivity were assessed. Paired tests and hospital-level mixed-effects models were used for exploratory comparisons of continuous outcomes. Results At baseline, 98 of 130 samples (75.38%) exceeded 10 CFU/100 mL; crude non-compliance was 84.62% (66/78) at AER outlets and 61.54% (32/52) at manual-rinse sinks ( P < 0.001). Identified organisms were predominantly water-associated bacteria, including Cupriavidus spp., Rhizobium spp., Stenotrophomonas maltophilia , and Moraxella osloensis . No culturable bacteria were recovered from any of the 260 post-intervention samples collected through day 90. Median endotoxin decreased from 0.0625 to 0.0265 EU/mL (Z = -3.485, P < 0.001). Conductivity was higher on day 7 than at baseline (median, 42.02 vs 14.25 µS/cm; P = 0.005), but the difference was not statistically significant on day 90 (15.98 µS/cm; P = 0.861). Conclusions Baseline microbial contamination of final rinse water supply pipelines is prevalent across tertiary hospitals in Shanghai, dominated by water-associated opportunistic pathogens. Continuous 2 mg/L HOCl treatment was associated with the absence of culturable bacteria in final-rinse water samples for 90 days and with lower endotoxin concentrations. This approach offers a feasible engineering intervention for continuous microbial control of final rinse water systems in endoscopy units.