Penicillin Blood-Level Determinations With a Streptococcus dysgalactiae Resistant to Normal Blood Inhibitors

A strain of Str. dysgalactiae was found to be an effective test organism for penicillin blood-level determinations. This organism, although inhibited by penicillin in concentrations of 0.006-0.008 unit/ml., is resistant to the natural inhibiting substances of blood sera. The latter characteristic is very significant, since the test organisms (Str. pyogenes C-203 and B. subtilis) that are now employed for penicillin assay of body fluids are inhibited by a large percentage of human sera. The method described can detect penicillin blood levels in concentration of 0.016 unit/ml. of blood sera. Since blood levels above 0.03 unit/ml. are considered to be therapeutically effective, this method of assay is adequately sensitive for routine clinical application.