Mechanism of cefoxitin and cephalothin interference with the Jaffé method for creatinine.

Some cepha antibiotics interfere positively in the Jaffé method for determining creatinine. The extent of this interference differs with the modification used, and its mechanism is unknown. We studied the spectrophotometric and thermodynamic properties of the reaction of picrate with creatinine and two cepha antibiotics, cefoxitin and cephalothin. The absorbance spectra of the chromophore produced by each of the three compounds when reacted with picrate were essentially identical, with an absorbance maximum at 485 nm. We determined the molar absorptivity, rate constant, and equilibrium constant for the reaction of the three compounds with picrate. The molar absorptivity was similar for creatinine and cefoxitin but about fourfold smaller for cephalothin. The rate constant and equilibrium constant for the reaction with picrate were respectively 20- and 10-fold greater for creatinine than for either of the two cepha antibiotics. Thus, these differences indicate the mechanism by which these two cepha antibiotics interfere with the Jaffé procedure for creatinine, and explain why the degree of interference depends on details of the procedure.