Rapid bioassay for clindamycin alone and in the presence of aminoglycoside antibiotics

A rapid bioassay for determination of concentrations of clindamycin in serum was developed with use of a strain of Lancefield group B Streptococcus (Streptococcus agalactiae) that is uniformly resistant to aminoglycoside antibiotics, tetracycline, and polymyxin. An agar diffusion assay system was used that included the addition of patients' sera and three standard concentrations of clindamycin to 5-mm wells cut in the seeded agar. Pretreatment of serum with penicillinase allowed measurement of clindamycin in the presence of penicillins and cephalosporins by the same assay method. Assays of clindamycin in serum using this system could be read routinely in as little as 4 hr and allowed determination of levels of drug in serum of 2.5-40 μg/ml. Linear regression analyses indicated that values obtained by this assay compared favorably with the results obtained with use of Bacillus subtilis strain ATCC 6633 or Sarcina lutea strain ATCC 9341 (American Type Culture Collection, Rockville, Md.). Repetitive measurement of sera with known concentrations of clindamycin indicated the average deviation to be ± 10%. Seeded bioassay plates could be prepared in advance and stored at 2 C-8 C for up to one week before use. This assay may also be used for measurement of lincomycin, erythromycin, vancomycin, and certain β-lactam antibiotics in the presence of aminoglycosides.