In many assays, hydrogen peroxide is employed as a reagent. When the assays are quantitative or semi-quantitative, it is necessary that the hydrogen peroxide react in the manner intended to produce a detectable signal, usually chromogen formation. In many assays, blood, or blood derivatives such as serum or plasma are present, where the blood sample contains a complex mixture of components. These components vary from individual to individual and, it is found can have an effect on the quantitation of hydrogen peroxide. It is therefore important to provide systems which prevent the reaction of hydrogen peroxide by pathways other than the desired assay pathway.