Ascorbate resistant composition, test device and method for detecting a component in a liquid test sample

In a composition for determining the presence of a component in a test sample, wherein said composition comprises a reagent system containing a peroxidatively active substance, and an oxidation-reduction indicator capable of providing a detectable response in the presence of H.sub.2 O.sub.2 and said peroxidatively active substance, and wherein said response is suceptible to being adversely influenced by the presence in said test sample of ascorbic acid, said composition further comprising a heavy metal compound which, in its ionized state, has an oxidation potential below that of said indicator, but above that of ascorbic acid,

• the improvement wherein said heavy metal compound is a complex of mercuric ion and one or more ligands, said ligand being covalently bound to said ion to form a water-soluble complex, and said ligand having a higher oxidation potential than said ion in its complexed state;
• said complex having a K.sub.s greater than about 10.sup.7, wherein K.sub.s is defined by the equation ##EQU6## and said complex being substantially noninterfering with respect to said reagent system.