What this test measures
This panel tests Mycobacterium tuberculosis grown on culture against 13 anti-TB drugs simultaneously to determine which the bug is sensitive to and which it is resistant to. The 13 drugs typically include first-line agents (streptomycin, isoniazid, rifampicin, ethambutol, pyrazinamide) and key second-line agents (kanamycin, capreomycin, levofloxacin, moxifloxacin, ethionamide, linezolid, clofazimine, PAS). The exact list varies slightly between labs.
The test is performed on a positive MGIT or LJ culture isolate. Phenotypic DST is the reference standard against which molecular tests (line probe assays, Xpert MTB/XDR) are validated, and it remains essential for non-first-line drugs and for confirming complex resistance patterns.
Why it matters
India has the largest absolute burden of multidrug-resistant TB (MDR-TB) in the world. NTEP's Programmatic Management of Drug-Resistant TB (PMDT) relies on DST results to design individualised treatment regimens that maximise the chance of cure while minimising toxicity. A comprehensive 13-drug DST matters in:
- Patients failing first-line treatment (suspected MDR / XDR). - Confirmed MDR-TB to guide which second-line drugs will work. - Pre-XDR or XDR-TB to design a salvage regimen. - Treatment monitoring for relapse or new resistance.
Molecular tests (CBNAAT, LPA) screen for the most common resistance mutations quickly, but they do not cover every drug. Phenotypic DST is slower (4–6 weeks) but tests the actual organism against the actual drug — making it indispensable for difficult cases.
How to prepare
A positive M. tuberculosis culture is required. If you do not yet have a culture, the lab will need a sputum (or other appropriate) sample for AFB MGIT culture first. Inform the lab of all current and past TB medications, so resistance patterns can be interpreted in context.
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.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Per-drug result (—)[1][2] | Sensitive | — | Resistant — the strain is resistant to this drug at the WHO-defined critical concentration. The drug should not be used in the regimen (or only after careful specialist review). Sensitive — the strain is inhibited at the critical concentration and the drug can be used. |
TB resistance categories
| Category | Definition | Treatment implication |
|---|---|---|
| Drug-sensitive (DS) | Sensitive to all 1st-line drugs | Standard 6-month regimen (HRZE → HR) |
| Isoniazid mono-resistant | Resistant to H, sensitive to R | Modified regimen with fluoroquinolone |
| Rifampicin-resistant (RR) | Resistant to R (treated as MDR) | PMDT MDR regimen |
| MDR-TB | Resistant to both H and R | PMDT longer or shorter MDR regimen |
| Pre-XDR | MDR + resistance to fluoroquinolone or injectable | Individualised regimen with newer drugs |
| XDR-TB | MDR + resistance to fluoroquinolone + injectable | Individualised regimen with bedaquiline / linezolid |
Frequently asked questions
Why test 13 drugs at once?
Drug-resistant TB regimens are designed by combining 4–6 drugs the bug is known to be sensitive to. Testing 13 drugs simultaneously gives the doctor enough options to build an effective and well-tolerated regimen — especially in MDR and pre-XDR TB.
How long does the result take?
Once a positive culture is available, phenotypic DST adds another 1–2 weeks for first-line drugs and up to 4 weeks for second-line drugs. Faster molecular tests (LPA, Xpert MTB/XDR) are run first to give earlier preliminary results.
Will the result change my treatment immediately?
Yes — DST results often lead to a change in regimen, especially if resistance to first-line drugs is found. NTEP's PMDT teams design and supervise these individualised regimens.
I am on TB treatment already — does that affect the test?
Inform the lab and your doctor. Ongoing treatment can affect culture yield but does not change DST interpretation if a positive culture is obtained.
Is this test free under NTEP?
NTEP provides DST free of charge at designated intermediate and national reference labs. Private testing is also available and may be needed when not eligible for the free pathway or when faster turnaround is required.
Are there safer drugs available now for MDR / XDR TB?
Yes — newer agents like bedaquiline, delamanid, pretomanid, and short-course regimens (BPaL / BPaLM) have transformed MDR / XDR TB outcomes. NTEP has rolled out many of these under PMDT. Your specialist will guide the right regimen.
How do I get this test under NTEP?
Speak to your DOTS centre or TB unit — they can refer you to an intermediate or national reference lab. Zelnoo can also facilitate private DST through NABL-accredited mycobacteriology labs.
Related Tuberculosis / Mycobacterial tests
Tests commonly ordered alongside AFB DRUG SUSCEPTIBILITY (13 DRUGS), or that help interpret an unexpected result.
Sources & references
- WHO Consolidated Guidelines on Drug-Resistant TB · accessed 2026-05-30T00:00:00.000Z
- India TB Report 2024 (Central TB Division) · accessed 2026-05-30T00:00:00.000Z
- NTEP Programmatic Management of Drug-Resistant TB Guidelines · accessed 2026-05-30T00:00:00.000Z
- CDC — TB Drug Susceptibility · accessed 2026-05-30T00:00:00.000Z
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