Test means, such as a composition or device, method of making a test device and process for determination of at least one oxidizing substance, such as a peroxide, are disclosed. More particularly, the contemplated test means comprises a hydrazone and 8-amino-1-napthol-5,7-disulfonic acid (Chicago acid). Further provided is a test system for the determination of a constituent in a sample, having means responsive to the presence of said constituent in said sample to produce at least one oxidizing substance and a composition for determining said at least one oxidizing substance, wherein said composition comprises a hydrazone and 8-amino-1-napthol-5,7-disulfonic acid (Chicago acid). The test system is preferably of the type which determines peroxides formed from enzymatic conversion of constituents in biological fluids. When in the form of compositions the test means can optionally be incorporated with a carrier, such as a tablet or matrix, to provide a test device. The test system is highly sensitive to low levels of body fluid constituents to be detected, while also being highly resistant to interfering reducing substances, such as ascorbic acid, often present in body fluids.