This procedure is adapted from the method of Folin and Wu described in Hawk, Oser & Summerson .
The method is based on the Jaffe reaction. Creatinine reacts with picrate ion formed in alkaline medium to develop a red-orange colour. The colour produced from the sample is then compared in a colorimeter at wavelength of 505 nm with that produced by a known amount of creatinine under the same condition.
The Jaffe reaction is known to be nonspecific when applied to the measurement
of creatinine in plasma. Many compounds, including protein, glucose,ascorbic
acid, acetone, and keto acids can also form the Jaffe reaction and cause overestimation
of the creatinine in plasma. Deproteinisation is an important step in creatinine
determination of plasma.
Why measure creatinine?
 HAWK,OSER, & SUMMERSON. Practical Physiological Chemistry (12th Edition).
The Blakiston Co. pp. 506-509.