Carbenicillin-gentamicin interaction in acute renal failure.

A case is reported in which carbenicillin inactivation of gentamicin resulted in subtherapeutic serum gentamicin levels in a 77-year-old man with acute renal failure. The drugs were administered separately to prevent in vitro inactivation. Carbenicillin inactivation of gentamicin resulted in subtherapeutic serum gentamicin levels (less than 2 microgram/ml) even with unusually large doses of gentamicin (1.8 mg/kg/day). The large dose of carbenicillin (15 g/day) resulted in serum carbenicillin levels more than 500 microgram/ml and an increased rate of gentamicin inactivation. After carbenicillin was discontinued, therapeutic serum gentamicin levels were achieved and maintained with doses of 0.9 - 1.8 mg/kg/day. It is suggested that if the combination of carbenicillin and gentamicin is used in patients with severe renal failure, the dose of carbenicillin should be reduced to prevent rapid inactivation of gentamicin. Further, to assure therapeutic gentamicin levels, serum levels of both drugs should be monitored frequently.