Improved method for specific determination of creatinine in serum and urine.

The Jaff#{233} reaction is commonly used to determine the concentration of creatinine in urine and serum by measuring the color produced when picric acid and sodium hydroxide solutions are combined with urine or protein-free filtrate from serum. As is well known, substances other than creatinine react to give positive error, the relative amount of interference usually being much greater in serum than in urine. Many specific methods have been proposed for determining creatinine in serum and urine. In several of these, creatinine is separated from interfering substances with strongly acidic cation-exchange resin (1-4). After the creatinine is adsorbed by the resin from an acid solution, it is easily eluted by increasing the pH. Most substances that reportedly interfere with the determination of creatinine by the Jaff#{233} reaction are neutral or anionic and would not be retained on a cation-exchange resin, and so would not interfere with these methods. In the methods of Teger-Nilsson (1), Sadilek (2), and Rockerbie and Rasmussen (4), serum or urine is diluted with weak acid and applied to the resin, which is in the hydrogen form. After adsorption of creatinine and before it is eluted, the resin is converted to the sodium form by washing with a dilute sodium chloride solution. This is necessary because hydrogen ions in the eluate interfere with the Jaff#{233} reaction. During conversion of the resin to the sodium form, creatinine can be eluted from the resin, causing poor recovery and incorrect results. We have developed a method in which the creatinine is separated from serum or urine with a small cation-exchange column in the sodium form at pH 3. After a water wash, the creatinine can be directly eluted with the Jaff#{233} reaction buffer. This rapid and