Effect of dialysis on interference by cefoxitin with determination of creatinine

Interference by cefoxitin with determination of creatinine is less with the Technicon SMAC than with other commercial analytical systems. The SMAC assay involves a single-point kinetic method with dialysis, whereas most other commercial methods are multipoint kinetic without dialysis. The apparent creatinine concentration measured for aqueous solutions of cefoxitin was 73 mmol of creatinine per mole of cefoxitin with the SMAC, 135 mmol/mol with a manual method. Furthermore, we determined for the SMAC that the average fraction of creatinine dialyzed was 0.128 and for cefoxitin, 0.064. Thus, the concentration of and interference by cefoxitin in the reaction mixture for SMAC are reduced by half (i.e., the apparent creatinine concentration for cefoxitin with the manual system multiplied by 0.5 is essentially that noted with the SMAC: 68 vs 73). Thus we conclude that the diminished interference be cefoxitin with determination of creatinine by SMAC is primarily ascribable to the dialysis step.