What tests determine susceptibility and guide regimen for suspected MDR-TB?

• A. HIV testing
• B. Chest X-ray
• C. Drug susceptibility testing (DST) using culture-based methods and molecular assays
• D. Sputum smear

Answer: (C)

Drug susceptibility testing is what tells you which anti-TB drugs will actually work against a patient’s bacteria, so it’s the key to guiding treatment when MDR-TB is suspected. DST determines the bacteria’s response to drugs by two main approaches: culture-based methods, which grow the organism in the presence of drugs and measure resistance (using solid media or liquid systems), and rapid molecular assays, which detect genetic mutations linked to resistance. Molecular tests can quickly flag rifampicin resistance and, with broader panels, other drug resistances, allowing a regimen to be tailored promptly rather than relying on a guesswork approach.

HIV testing, chest X-ray, and sputum smear each provide important clinical information, but they do not reveal which drugs the TB bacteria are susceptible to. DST directly informs the choice of drugs, making it the best tool for determining susceptibility and guiding therapy in suspected MDR-TB.