Fifty specimens from 27 infants and children with septic joint disease were assayed for ampicillin (18 specimens), methicillin (14 specimens), penicillin (14 specimens) or cephalothin (four specimens). The ranges of observed values varied depending upon the dose and the interval after administration. The joint-fluid level was the same as or higher than the serum level in eight of nine paired specimens tested for ampicillin and in seven of 11 paired specimens assayed for methicillin. Comparable serum and joint-fluid levels of penicillin were found in 11 of 12 paired specimens. The four antibiotics studied enter joint fluid in concentrations greatly in excess of in vitro inhibitory levels of the usual bacteria that cause septic arthritis. There is no need for intra-articular administration. Intramuscular therapy appears as effective as the intravenous route with ampicillin and probably with methicillin and penicillin.
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