TRINDER reagent is an in vitro chemical reagent that is critical in medical laboratory diagnostics. It is used for the rapid quantification of salicylates in biological fluids and for the analysis of various substrates and metabolic end products. Due to the catalytic action of ferric nitrate nonahydrate in combination with mercuric (II) chloride, a characteristic purple color is produced in the presence of salicylic acid. The color intensity, measured spectrometrically, correlates directly with the concentration of the analyte. In addition, the TRINDER reagent is used in the Trinder method, in which hydrogen peroxide formed by enzymatic reaction of the analyte (e.g., oxidation of glucose with glucose oxidase) reacts further with phenol and 4-aminophenazone under the action of peroxidase to form a color complex and water. The Trinder method is versatile and can be modified to determine uric acid, creatinine and cholesterol, for example.