Clinical Chemistry, Vol 11, 763-770, Copyright © 1965 by the American Association for Clinical Chemistry

"True" Creatinine Chromogen Determination in Serum and Urine by Semi-automated Analysis

¹ Department of Pediatrics, Chicago Medical School, Chicago, Ill., and Division of Pediatrics, Michael Reese Hospital and Medical Center, Chicago 16, Ill.

The Jaffè reaction, used to determine creatinine in urine and plasma or serum, gives a positive error in the latter because of the interference of noncreatinine. In man (particularly children) endogenous creatinine normally occurs at a relatively low concentration in serum (0.3-1.0 mg./100 ml.). The analytic error contributed by noncreatinine chromogens, therefore, may be considerable.

Of various manual methods to eliminate this none has been used with automated creatinine analyses, which generally measure total creatinine-like chromogen. In the present investigation a strong cation-exchange resin is used manually for rapidly adsorbing and desorbing "true" creatinine from plasma which is then analyzed by an automated method. A phosphate buffer (pH 12.4) is substituted for 5% (w/v) NaOH, dialysis eliminated, and the instrument manifold design is modified. Concentrations of "true" creatinine from 0.2-2.0 mg./100 ml. can be determined at a rate of 20 samples per hr. in serum or 40/hr. in urine, with a recovery of 98-101%.