Concentration of antibacterial agents in interstitial tissue fluid

The concentration of antibacterial agents in the interstitial tissue fluid has been studied in an experimental model using implanted perforated Silastic capsules (tissue cages). Tissue fluid concentrations were always lower than the initial peak concentration in the serum, but for those drugs that were more slowly excreted the tissue fluid was similar to the serum concentration after six hours. In contrast the concentration in the tissue fluid for those drugs that are excreted rapidly was unpredictable, being either negligible or maintained at concentrations significantly better than in the serum. There was no evidence of accumulation in tissue fluid with regular dosage. These results indicate that the tissues may never be exposed to an adequate concentration of some antibacterial agents. This may be of clinical importance in tissue infections. Antibiotics which are rapidly excreted should be given in large enough doses and often enough for adequate blood levels to be maintained, so allowing equilibrium with tissue fluid to be achieved.