Clinical significance of tissue penetration of antibiotics in the respiratory tract.

Concentrations of antibiotics in bronchial secretion vary widely and cannot be easily related to physico-chemical characteristics. Serum and sputum concentrations do broadly correlate in the case of penicillins, some cephalosporins and some tetracyclines. Antibiotic concentrations in bronchial secretion (although not necessarily in lung tissue) are always much lower than in serum, with a ratio of about 20% for tetracycline, 3--4% for ampicillin, 4--50% for gentamicin and tobramycin. Concentrations will often be adequate to inhibit highly sensitive pathogens but marginal or ineffective for those less sensitive, e.g. carbenicillin or gentamicin for Ps. aeruginosa.