Abstract: The invention provides an assay method for determining a level of haptoglobin in a sample comprising the steps of: (i) mixing haemoglobin with the sample to be assayed so as to form a haptoglobin-haemoglobin complex with haptoglobin present in the sample; (ii) contacting the product of step (i) with reagents for generating hydrogen peroxide and one or more chromogens which undergo a spectroscopically detectable change when peroxidase activity is present, in the presence of a buffer, under conditions in which hydrogen peroxide is generated from said reagents and forms a substrate for the peroxidise activity of the haptoglobin-haemoglobin complex present, and wherein the pH of the buffer is within a range which is sufficiently low that the peroxidise activity of any uncomplexed haemoglobin is substantially suppressed but sufficiently high that hydrogen peroxide generation occurs; (iii) determining the peroxidase activity of the haptoglobin-haemoglobin complex by measuring the change in an optical property of th