Interference of selected second- and third-generation cephalosporins with creatinine determination

The interference of selected second- and third-generation cephalosporins and cephalosporin-like antibiotics with serum and urine creatinine determinations was studied. Single determinations of apparent creatinine concentrations in serum, urine, and water samples were measured by three methods--manual, automated dialysis (SMAC 12/60), and direct kinetic (KDA Analyzer). Various concentrations of each cephalosporin were tested for interference with creatinine determinations. Drugs studied were cefoxitin, cefamandole, cefotaxime, ceftazidime, cefoperazone, ceftizoxime, moxalactam, ceftriaxone, and ceforanide. Cephalosporins demonstrating interference with one or more methods were further tested at concentrations representing the range of clinically achievable serum and urine drug concentrations. Cefoxitin, ceforanide, moxalactam, ceftizoxime, and ceftriaxone demonstrated significant interference in the screening tests. In subsequent testing, interference was shown at clinical concentrations of cefoxitin (all three methods) and ceforanide (manual and KDA methods). Where interference occurred, the degree varied with the assay method and the drug concentration. Recommendations are presented for serum and urine sampling for creatinine determinations in patients receiving cefoxitin and ceforanide. Based on the relationship between test interference and concentrations of these drugs achievable in patients with varying degrees of renal dysfunction, only cefoxitin and ceforanide can be expected to cause significant false elevations in vivo. To minimize interference, blood samples should be drawn at the time of minimum drug concentration.