The principle of the measurement of creatinine is based on the Jaffe reaction. That is, under alkaline conditions creatinine reacts directly with picric ions forming a reddish complex, the absorbance of which can be measured at 520 nm. However, several interfering substances including proteins, ketones, glucose, and ascorbic acid also react with picric acid, producing similar colored complexes. Serum proteins are precipitated with tungstic acid solution before measuring for creatinine.