Vesicular microreactors have gained broad interest in drug delivery, biodetoxification, and green chemistry. We have expanded their use to diagnostic applications by leveraging the selective permeability of the vesicular membrane. In the past, we developed a transmembrane pH-gradient polymeric microreactor to sense ammonia, a widely used biomarker in liver disease. After diffusing across the membrane, ammonia is protonated in the acidic lumen of the polymersome. The pH increase is detected by a pH-sensitive near-infrared fluorophore in the lumen. The high ammonia selectivity of this polymersome microreactor relies on the highly hydrophobic membrane of poly(styrene)-b-poly(ethylene glycol) polymersomes. In this study, we are combining ammonia-sensing polymersomes with a highly selective ammonia-generating enzyme, urease, to expand the analyte space and enable urea sensing in whole blood. Blood urea is a widely used biomarker in kidney disease, notably to determine the adequate duration of hemodialysis. In clinical routine, blood urea measurements are performed in centralized laboratories. A bedside test would enable real-time urea monitoring during hemodialysis with the potential to reduce the risk of over- and underdialysis. We first optimized the assay components and parameters (PS-b-PEG polymersomes, pH-sensitive dye, urease, incubation time and temperature) to optimize the sensor response and kinetics in phosphate buffer at pH 7.4. The urease-coupled polymersome assay was subsequently tested in urea-spiked fresh mouse blood. We observed a rapid and linear response at clinically relevant urea concentrations. Based on these results, the assay was tested in an IRB-approved study in healthy volunteers. In fresh capillary blood, the assay was able to discriminate three clinically relevant spiked urea concentrations in under one minute. Therefore, coupling the urease-catalyzed hydrolysis of urea with ammonia-sensing polymersomes yielded a blood urea assay with high selectivity and a rapid response at clinically relevant concentrations. These results highlight the potential of combining a highly selective ammonia-generating enzyme with ammonia-sensing polymersome microreactors for blood metabolite sensing at the point-of-care.