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U.S. Department of Health and Human Services

Mycobacteriology Laboratory Branch

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Drug susceptibility testing

Drug susceptibility testing is a difficult procedure to standardize, and proficiency in performing these tests requires an understanding of many elements, including the

  • Origin of drug resistance and the criteria for resistance
  • Potency and stability of drugs during laboratory manipulation
  • Antimycobacterial activity of drugs when incorporated into different media
  • Reading, interpretation, and reporting of results

The MLB uses an indirect proportion method for testing M. tuberculosis and other slow-growing mycobacteria to 11 drugs at 35°C on Middlebrook 7H10 agar. The test requires 1 month to complete.  Growth on the control medium is compared to the growth on the drug-containing medium to determine susceptibility or resistance. When performed properly, this method allows a quantitation of the proportion of mutants resistant to a drug and can detect the 1% proportion of drug- resistant mutants above which therapeutic failure is likely.

Slow growing mycobacteria are tested with the following drugs:

  • Isoniazid
  • Streptomycin
  • Ciprofloxacin
  • Capreomycin
  • Kanamycin
  • Pyrazinamide
  • Cycloserine
  • Ethambutol
  • Rifampin
  • Ethionamide
  • Rifabutin

Last Reviewed: 05/18/2008
Content Source: Division of Tuberculosis Elimination
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention


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