PS-915 dihydrochloride is a peptide substrate used in a colorimetric assay for plasma antithrombin III (ATIII). PS-915 dihydrochloride is highly specific for thrombin. By enzyme hydrolysis, PS-915 dihydrochloride liberates 3-carboxy-4-hydroxyaniline (CHA), which turns blue in color due to the complex formation with added alkaline-pentacyanoammine ferroate.