Penicillin-aminoglycoside inactivation: another possible mechanism of interaction

A case of a carbenicillin-tobramycin interaction resulting in laboratory test reports of subtherapeutic serum tobramycin levels in a 71-year-old man with renal failure is reported. The patient's Pseudomonas aeruginosa infection was treated with carbenicillin, 2 g every eight hours, and tobramycin, 80 mg after daily hemodialysis. The serum antibiotic levels were monitored using a microbiologic assay and a radioimmunoassay technique. At 30 minutes and five hours after the tobramycin was administered, microbiologic assay of serum levels indicated negligible tobramycin concentrations (2 micrograms/ml). Radioimmunoassay of tobramycin levels showed markedly higher concentrations (4.1 micrograms/ml at 30 minutes after infusion). The difference in assay results was attributed to greater inactivation occurring with the microbiologic assay, which was the less rapid technique. In vitro and in vivo factors influencing the occurrence and extent of the carbenicillin-tobramycin interaction are reviewed. When using an aminoglycoside-penicillin combination in patients with renal failure, it is important to use a rapid assay technique or one that inactivates one of the antibiotics before bioassay, and to be aware of the patient-related factors that can alter assay results.