Cefoxitin falsely elevates creatinine levels.

Cefoxitin sodium has been found to act like creatinine in assays for creatinine used by many hospitals. Our in vitro results show that cefoxitin causes an apparent rise in the serum creatinine level; the magnitude of the increase depends on the assay method and the concentration of cefoxitin. Results varied from 1.5 to more than 8.5 times the normal serum creatinine level. Published data from studies of human volunteers show the same effect. In patients with normal renal function, the serum creatinine test is reliable two to four hours following cefoxitin infusion and in mild to moderate renal failure (glomerular filtration rate greater than 30 mL/min), six to eight hours. In severe renal failure, the serum creatinine determination is unreliable. Measurement of urinary creatinine employs similar assays, thus making estimation of creatinine clearance unreliable when the patient is receiving cefoxitin.