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

The Jaff#{232} reaction, used to determine creatinine in urine and plasma or serum, givesa positiveerror in the latter becauseof the interference of noncreatinine. In man (particularly children) endogenouscreatinine 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 phosphatebuffer (pH 12.4) issubstituted for 5% (w/v) NaOH, dialysisis 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 recoveryof 98-101%.