Diagnostic agent suitable for use in carrying out rapid analytical determinations of the presence and/or concentration of hydroperoxides, substances which react with the liberation of hydrogen peroxide, peroxidase or peroxidate active substances, comprising a chromogen which is oxidized by hydroperoxide in the presence of peroxidase or peroxidate active substances to form a dyestuff, the color intensity of which is dependent on the quantity of peroxide, peroxidase or peroxidate active substance present in the test sample, wherein the chromogen is a compound having the formula: WHEREIN Y is hydroxyl, mercapto, or amino, Z is hydroxyl, wherein Y AND Z together can represent oxygen, and R is hydrogen, mercapto, amino, lower alkyl, pyridino, morpholino, piperazinyl, arylpiperazinyl, phenyl, or phenyl substituted by one or more of hydroxyl, nitro, alkyl or alkoxyl.