ANTIBIOGRAM - MTB (ETHIONAMIDE)

This test determines whether a cultured M. tuberculosis isolate is sensitive or resistant to ethionamide at the WHO critical concentration. Ethionamide (Eto) and prothionamide (Pto) are thioamides that inhibit mycolic acid synthesis — the same broad pathway as isoniazid. They are used as second-line companion drugs in MDR-TB regimens.

Resistance is mediated by mutations in inhA (often shared with low-level isoniazid resistance) and ethA. inhA promoter mutations therefore confer cross-resistance between isoniazid and ethionamide — a clinically important finding.

Knowing ethionamide susceptibility helps the specialist decide whether to include Eto in a longer MDR-TB regimen. Where inhA promoter mutations are known (from LPA), ethionamide is usually avoided. Ethionamide has a difficult tolerability profile — nausea, vomiting, hepatitis, hypothyroidism, peripheral neuropathy — which limits its use, especially in patients on co-medications.

In India, ethionamide remains part of certain longer MDR-TB regimens despite all-oral regimen progress, particularly when other companion drugs are not options.

A positive M. tuberculosis culture is required. If not yet available, sputum samples must be collected for AFB culture first. Mention any previous isoniazid use or LPA result (inhA status) to your doctor.