the indicator dye thereby initiating the color transition of the developer coupler combination a suitable hydroperoxide should be sufficiently stable such that free oxygen is not liberated in the absence of a peroxidatively active substance and should possess a sufficiently low vapor pressure such that it does not evaporate or sublime from the reagent composition during storage or after it is incorporated into a carrier matrix of a dry phase test strip furthermore when the reagent composition is to be used in the assay of urine for occult blood the hydroperoxide should demonstrate a sufficient sensitivity to detect 1 part of hemoglobin in one million parts of the test sample suitable hydroperoxides include cumene hydroperoxide t butyl hydroperoxide diisopropylbenzene hydroperoxide 1 hydroxycyclohexane 1 hydroperoxide 2 5 dimethylhexane 2 5 dihydroperoxide paramenthane hydroperoxide 1 4 diisopropyl monohydroperoxide p t butylisopropylbenzene hydroperoxide 2 u03b1 hydroperoxyisopropyl 6 isopropylnaphthalene tetralin hydroperoxide and combinations thereof in the assay of urine for occult blood 1 4 diisopropylbenzene dihydroperoxide dbdh is the preferred hydroperoxide because of its stability sensitivity and non volatility the concentration of hydroperoxide in the indicator reagent composition will normally range from about 1 mm to about 100 mm and from 100 to 10 000 mg m 2 in the matrix format preferred concentrations range from 25 to 75 mm and 2 500 to 7 500 mg m 2 the specific amount of a particular hydroperoxide included in the composition is dependent upon the physical and chemical properties of the particular hydroperoxide such as stability and its sensitivity towards the peroxidatively active substance being assayed while the invention is not to be limited to any.