Serum penicillin concentrations after intramuscular administration of benzathine penicillin G in children with rheumatic fever and controls.

Serum and urine penicillin levels were determined in 11 children with rheumatic fever (RF) who were receiving benzathine penicillin G (BPG) prophylactically every 3 weeks and in 10 children without RF who received the drug for the treatment of other infections. The dose given was 600,000 units for children weighing less than 25 kg and 1,200,000 units for those with a weight above 25 kg. Blood and urine samples were collected from both groups before and on days 7, 14 and 21 after BPG administration. Our results showed that: minimum inhibitory concentrations (MICs) of BPG for group A beta-hemolytic streptococci were 0.02 IU/ml or 0.0125 microgram/ml; intramuscular BPG did not give adequate serum levels to block the growth of group A beta-hemolytic streptococci in approximately 24 and 62% of children included in the study on days 14 and 21 after its administration, respectively; BPG metabolism was similar in both groups and did not depend on the underlying disease; serum and urine levels did not vary according to sex and weight; and there was a small correlation between serum and urine levels.