Pseudomonas aeruginosa in hospital water is a risk for invasive infection. Point-of-use (POU) filters are used to reduce patient exposure to the organism, and hollow-fibre filters are becoming more popular. However, retrograde colonization of the filter mechanism may contaminate the effluent.

To assess the efficacy of POU filter head (polysulfone; hollow-fibre matrix) shower filters in preventing the exposure of high-risk patient groups to P. aeruginosa.

Pre-flush (opening the outlet and collecting the first 100 mL of water) samples were analysed to measure P. aeruginosa contamination from 25 shower outlets (∼21% of all showers on the six wards), with and without a hollow-fibre filter. P. aeruginosa was measured in a subset of outlets harbouring P. aeruginosa (sampling period 19^th August 2019 to 10^th January 2020).

Water from all 25 showers was heavily colonized [>300 colony-forming units (cfu)/mL] with P. aeruginosa at the showerhead. P. aeruginosa was found in 32% (8/25) of post-filter shower water effluent samples with a geometric mean of 4x10⁶ cfu/mL (N=4) (6.8x10⁴–2x10⁸). Filters were sampled at 15–150 days of use (median 15 days), with 26% (6/23) of filter units becoming colonized before the expiry date.

POU filter showerhead units may not be effective in preventing exposure of vulnerable patients to P. aeruginosa in hospital water due to retrograde contamination (external contamination of the showerhead passed back to the filter cartridge itself) or failure of the hollow-fibre filter matrix. Reliance should not be placed on the use of hollow-fibre filters to protect patients from exposure to P. aeruginosa without repeated microbiological monitoring.