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 LB uses an indirect proportion method for testing M. tuberculosis complex mycobacteria to 12 drugs at 35C 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.

M. tuberculosis complex are tested by this method with the following drugs:

• Isoniazid
• Rifampin
• Ethambutol
• Ciprofloxacin
• Ofloxacin
• Streptomycin
• Kanamycin
• Capreomycin
• Amikacin
• Rifabutin
• Ethionamide
• Para-aminosalicylic acid (PAS)

Pyrazinamide is tested by the MGIT 960 method.  

Additionally, bedaquiline testing is available upon request [PDF 108 KB]

Additional Resources

• Report of Expert Consultations on Rapid Molecular Testing to Detect Drug-Resistant Tuberculosis in the United States