What this test measures
This test determines whether a cultured M. tuberculosis isolate is sensitive or resistant to ethambutol at the WHO-defined critical concentration. Ethambutol (E or EMB) is one of the four core first-line TB drugs (alongside isoniazid, rifampicin, pyrazinamide). It inhibits arabinosyl transferase, blocking mycobacterial cell-wall synthesis.
Resistance is mediated mostly by mutations in the embB gene. Phenotypic DST is reliable for ethambutol when performed on MGIT or LJ at the WHO critical concentration; some discrepancies with molecular tests occur.
Why it matters
Ethambutol is included in the intensive phase of the standard 6-month TB regimen (HRZE) primarily to prevent resistance amplification of the other companion drugs while susceptibility results are awaited. Confirmed ethambutol resistance changes regimen composition — particularly in retreatment and MDR-TB scenarios.
Ethambutol has a distinctive side effect: dose-related optic neuritis (loss of colour vision, blurred vision, reduced visual acuity), which is usually reversible if caught early. Visual symptoms during treatment require prompt evaluation and may need drug discontinuation.
How to prepare
A positive M. tuberculosis culture (MGIT or LJ) is required. If not available, sputum samples must be collected for AFB culture first.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
First-line TB drugs at a glance
| Drug | Target | Distinctive toxicity |
|---|---|---|
| Isoniazid (H) | Mycolic acid synthesis | Hepatitis, neuropathy |
| Rifampicin (R) | RNA polymerase | Hepatitis, orange urine, drug interactions |
| Pyrazinamide (Z) | Acid pH activity | Hepatitis, hyperuricaemia |
| Ethambutol (E) — this drug | Cell wall (arabinosyl) | Optic neuritis (reversible) |
Frequently asked questions
What is ethambutol used for?
Ethambutol is part of the standard 6-month first-line TB regimen (HRZE in the intensive phase) and is also used in selected NTM and MDR-TB regimens.
Why does ethambutol affect the eyes?
Ethambutol can cause optic neuritis — usually dose-dependent, often reversible if caught early. Patients should report any vision change (blurring, loss of red-green colour) immediately.
How accurate is ethambutol DST?
Phenotypic DST at the WHO critical concentration is generally reliable. Some borderline results occur; molecular testing (embB mutations) is a useful supplement.
Will ethambutol resistance change my treatment?
Yes — ethambutol is dropped from the regimen and replaced by another drug guided by the full susceptibility profile and your clinical scenario.
How long does the test take?
About 1–2 weeks from a positive culture; 4–6 weeks total from sputum collection.
Is the test available under NTEP?
Yes — first-line DST is offered at NTEP intermediate and national reference labs. Private NABL-accredited labs also offer it.
Do I need an eye check during TB treatment?
A baseline visual assessment is recommended before starting ethambutol, with prompt re-evaluation if any visual symptom develops.
Related Tuberculosis / Mycobacterial tests
Tests commonly ordered alongside ANTIBIOGRAM - MTB (ETHAMBUTOL), or that help interpret an unexpected result.
Sources & references
- WHO Consolidated Guidelines on Drug-Resistant TB · accessed 2026-05-30T00:00:00.000Z
- NTEP PMDT Guidelines · accessed 2026-05-30T00:00:00.000Z
- CDC — TB Laboratory Diagnostics · accessed 2026-05-30T00:00:00.000Z
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