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We describe a novel infection-responsive coating for urinary catheters that provides a clear visual early warning of Proteus mirabilis infection and subsequent blockage. The crystalline biofilms of P. mirabilis can cause serious complications for patients undergoing long-term bladder catheterisation. Healthy urine is around pH 6, bacterial urease increases urine pH leading to the precipitation of calcium and magnesium deposits from the urine, resulting in dense crystalline biofilms on the catheter surface that blocks urine flow. The coating is a dual layered system in which the lower poly(vinyl alcohol) layer contains the self-quenching dye carboxyfluorescein. This is capped by an upper layer of the pH responsive polymer poly(methyl methacrylate-co-methacrylic acid) (Eudragit S100®). Elevation of urinary pH (>pH 7) dissolves the Eudragit layer, releasing the dye to provide a clear visual warning of impending blockage. Evaluation of prototype coatings using a clinically relevant in vitro bladder model system demonstrated that coatings provide up to 12 h advanced warning of blockage, and are stable both in the absence of infection, and in the presence of species that do not cause catheter blockage. At the present time, there are no effective methods to control these infections or provide warning of impending catheter blockage., Graphical abstract A pH-responsive diagnostic coating for urinary catheters is presented, to provide advance warning of encrustation and blockage by P. mirabilis. Diagnosis is facilitated by the stimulated release of 5(6)-carboxyfluorescein from a hydrogel reservoir in response to increased urinary pH. Catheter prototypes are tested using a clinically relevant continuous flow in vitro bladder model system, and artificial urine. fx1, Highlights • Development of a dual-layered diagnostic coating sensor for urinary catheter. • Provides advance warning of infection by P. mirabilis in response to alkaline urine. • Read out via release of carboxyfluorescein dye from coating into urine leg bag. • Catheter prototypes ‘report’ infection 12 h before blockage by P. mirablis.