Serum Unbound Levels of Cloxacillin and Erythromycin in Cystic Fibrosis

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Abstract
Cloxacillin and erythromycin were absorbed normally by children with cystic fibrosis. In individuals from whom staphylococci were isolated during continuous antibiotic therapy, the serum unbound level, the biologically active fraction, did not reach the level required to inhibit the growth of the patient's staphylococci in vitro. This suggests that if staphylococci are isolated from the respiratory tract of a child during long-term chemotherapy, increasing the dose may eliminate the pathogen. The minimum doses of cloxacillin to produce adequate serum unbound levels appear to be 250 mg. for children below 20 kg. and 500 mg. above this weight. The effective dosage for erythromycin is less predictable, but a similar regimen is probably adequate.