Drug Susceptibility Testing (DST) determines the ability of several anti-tuberculosis drugs to inhibit the growth of Mycobacterium tuberculosis complex (MTBC). DST is very important to assess the correct drug regimen for patients with tuberculosis.

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

• 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 CDC Laboratory Branch 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)

In addition, pyrazinamide is tested by the MGIT 960 method (Becton Dickinson).