Antibiotic assays in muscle: are conventional tissue levels misleading as indicator of the antibacterial activity?

Concentrations of ampicillin and cefuroxime in serum and muscle were studied in rabbits following a single intramuscular injection of 25 mg/kg. Simultaneous measurements were performed in whole muscle tissue pieces and in muscle tissue fluid obtained by implantation of cotton threads under the muscle fascia. The concentrations in the tissue fluid showed only a short time lag to thea serum levels. However, levels in whole muscle were substantially lower than the corresponding concentrations in muscle tissue fluid, indicating that the antibiotics were not evenly distributed within the tissue. The whole muscle tissue levels seriously underestimated, by a factor of approximately 5 times, the actual antibiotic concentrations in the extracellular fluid, where the microorganisms causing soft tissue infections are most likely to be found.