Rapid radiometric susceptibility testing of Mycobacterium tuberculosis

A 48-hour radiometric test for determining the drug susceptibility of Mycobacterium tuberculosis has been developed. The test is based on the measurement of ¹⁴CO₂ produced by the oxidation of formate labeled with carbon-14. The test system uses 5 x 10⁷ organisms in 1 ml of Middlebrook 7H9 medium plus albumin-dextrose-catalase enrichment and 1 μCi of [¹⁴C]formate. The ¹⁴CO₂ produced is measured in an ionization chamber at 24-, 48-, and 72-hour intervals, with and without the addition of antituberculous drugs. Isoniazid, streptomycin, rifampin, and ethambutol were each tested at 3 concentrations by the radiometric method and the reference (agar dilution) method. Six standard strains and 21 patient isolates were compared by both methods. Production of ¹⁴CO₂ was quantitatively decreased in the presence of drugs that inhibit the organism. The radiometric method requires 2 days; the agar dilution, 14 to 21 days.