Serum Levels of Penicillin and their Variations after Administration of Benzathine Penicillin in Children

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Abstract
The serum penicillin level of 131 children aged 5 to 15 years, following injection of varying doses of benzthine penicillin, was studied. Although the median level was felt to be treponemicidal for 14 to 18 days after 600,000 units and for 28 days after 1.2 million units, considerable variation in levels between patients made it impossible to be sure of a treponemicidal level in a given patient. Nine to 13 days after an injection of 600,000 units serum levels ranged from zero to 0.388 units 1 ml. Similar variations were seen in a single patient. One patient studied 3 times after receiving 600,000 units of benzathine penicillin and 600,000 units of "bipenicillin" showed zero level at 14 days following the 1 two tests, and 0.35 units following the 3d test. Unexplained irregular levels were also seen. On patient showed 0.76 units of penicillin on the 8 day after receiving 1.2 million units of benzathine penicillin and on the 14 day showed 17 units. A relationship could not be made between physical activity and serum penicillin levels. These irregularities may result from sudden liberation of penicillin from the depot in the muscle under circumstances which cannot be determined clinically. In patients in whom the serum levels remained at zero over a period of days after injection, the injected penicillin may have become encysted and rendered inert in the muscle. The anomolous blood levels reported here may explain some syphilitic treatment failures.