Oxidative stress (OS) is a universal feature of acute and critical illness. It occurs when pro-oxidants overwhelm antioxidant defences. There is currently no easy bedside measurement of OS. It is widely recognised that a simple, real-time, point-of-care measurement of oxidative stress could be of significant benefit in the assessment and management of patients with acute and critical illness. The solution we propose is a method called cyclic voltammetry which can measure the key components of blood total antioxidant status in one simple measurement cycle that takes only a few minutes. We have tested this in animal models and now propose to optimise the method for human samples, and to validate it in a range of patients in an observational study. The development of a bedside oxidative stress test has far reaching application both clinically and in research to predict and follow changes in OS during illnesses or potentially to guide therapy.