Rapid bioassay for chloramphenicol in the presence of other antibiotics

A simple, rapid bioassay for the measurement of chloramphenicol in serum or cerebrospinal fluid was developed using a multiply antibiotic-resistant strain of Escherichia coli. The agar diffusion system involved the addition of patients' specimens and three standard concentrations of chloramphenicol to 7.5 mm diameter wells cut in agar seeded with the test organism. Assays of chloramphenicol using this system could be read routinely in three to four hours and allowed determinations of levels of 5-60 μg/ml. Chloramphenicol could be measured accurately in the presence of a variety of beta-lactam, aminoglycoside, sulfonamide, and tetracycline antimicrobial agents, but not cefoxitin or trimethoprim-sulfamethoxazole. Repetitive assays for sera containing known concentrations of chloramphenicol indicated a coefficient of variation of 8%. Seeded assay plates could be stored at 2-8°C for up to five days prior to use.