Abstract: The concentration of NADH or NADPH in a test solution is determined by adding a redox coupling agent, preferably 2,6 dichloroindophenol DCIP, to the test solution. The coupling agent reacts with the NADH or NADPH to form an electroactive coupling agent (DCIPH.sub.2) which is then detected electrochemically at a lower voltage than would be required to detect the NADH or NADPH. This can be used to detect NADH or NADPH formed by any well known enzymatic or immunoassay method which produces NADH as a detectable product. This has particular application to biological fluids such as whole blood which does not require any treatment of the test sample prior to electrochemical analysis. In particular, red blood cells do not have to be removed from whole blood samples to provide reliable data.