ANTIBIOGRAM - MTB (RIFABUTIN)

This test determines whether a cultured M. tuberculosis isolate is sensitive or resistant to rifabutin at the WHO-defined critical concentration. Rifabutin is a rifamycin similar to rifampicin but with fewer drug interactions, making it preferred in patients on certain antiretroviral therapy (ART) regimens — notably protease inhibitors.

Most rifampicin-resistant strains are also rifabutin-resistant because the same rpoB mutations confer cross-resistance. However, certain low-level rpoB mutations (e.g. D516Y) may retain rifabutin susceptibility — so rifabutin DST is sometimes useful in selecting an alternative rifamycin.

For Indian patients living with HIV who develop TB, choosing the right rifamycin matters. Rifampicin strongly induces protease inhibitor metabolism and reduces antiretroviral levels significantly. Rifabutin has weaker enzyme-induction effects and is often preferred when a protease inhibitor cannot be substituted.

Rifabutin susceptibility testing therefore matters particularly in HIV-TB co-infected patients, in those with rifampicin-resistant isolates where some rifamycin activity might be preserved, and in patients with intolerance to rifampicin.

A positive M. tuberculosis culture (MGIT or LJ) is required. If not yet available, sputum samples must be collected for AFB culture first. Inform the lab of any HIV status and antiretroviral regimen for context.