What this test measures
The first-line LPA (e.g. Hain GenoType MTBDRplus) is a molecular test that detects M. tuberculosis DNA and the most common resistance mutations to the two key first-line drugs — isoniazid (katG, inhA promoter) and rifampicin (rpoB). DNA is amplified by PCR, then hybridised to a strip carrying wild-type and mutation-specific probes. Coloured bands on the strip indicate which mutations are present.
LPA gives a result within 1–2 days from a smear-positive sample (or from a positive culture for smear-negative cases). It identifies both the bacterium and the resistance pattern simultaneously.
Why it matters
Detecting isoniazid resistance in addition to rifampicin resistance is a critical advantage over CBNAAT (which detects only rifampicin). For India's rising H-mono-resistant TB (about 1 in 10 new cases), LPA distinguishes:
- Drug-sensitive TB → standard HRZE regimen. - H-mono-resistant TB → 6 RZE + Levofloxacin (WHO regimen). - RR-TB / MDR-TB → PMDT pathway. - High-level vs low-level H resistance → guides ethionamide eligibility.
LPA is part of NTEP's algorithm at intermediate and reference labs alongside CBNAAT. The result is faster than phenotypic DST and informs early regimen choice.
How to prepare
Smear-positive sputum (preferred) or positive culture isolate. If smear-negative, samples need to be cultured first. Inform the lab of any TB drug already taken.
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 |
|---|---|---|---|
| M. tuberculosis DNA (—)[1] | Not detected | — | Detected — confirms TB. Move to resistance interpretation. |
| Rifampicin resistance (rpoB) (—) | Wild-type — sensitive | — | Mutation detected — rifampicin resistance. Treat as MDR-TB; PMDT pathway. Send isolate for second-line DST. |
| Isoniazid resistance (katG / inhA) (—)[1] | Wild-type — sensitive | — | katG mutation — high-level isoniazid resistance; ethionamide usually still works. inhA promoter mutation — low-level isoniazid resistance with ethionamide cross-resistance. Both alter regimen choice. |
First-line LPA vs CBNAAT vs MGIT DST
| Feature | CBNAAT | LPA (this test) | MGIT DST |
|---|---|---|---|
| Detects MTB | Yes | Yes | Yes |
| Rifampicin resistance | Yes | Yes (specific mutation) | Yes (phenotypic) |
| Isoniazid resistance | No | Yes | Yes |
| Distinguishes katG vs inhA | No | Yes | No |
| Turnaround | 2 h | 1–2 days | 1–2 weeks (after positive culture) |
Frequently asked questions
How is LPA different from CBNAAT?
CBNAAT detects only rifampicin resistance; LPA detects both isoniazid and rifampicin resistance, and distinguishes the mutation type (which informs ethionamide cross-resistance). LPA takes a little longer and needs more lab infrastructure.
What does it mean if I have a katG mutation?
High-level isoniazid resistance. Ethionamide typically still works because it acts through a different pathway.
What does an inhA promoter mutation mean?
Low-level isoniazid resistance with ethionamide cross-resistance — important when designing MDR-TB regimens.
When is LPA done?
For confirmed TB cases needing fast drug-resistance information, especially when CBNAAT shows rifampicin resistance or isoniazid resistance is clinically suspected. Used widely in NTEP's drug-resistance survey too.
How long does the test take?
Approximately 1–2 days from a smear-positive sample. From smear-negative samples, the bacteria need to be cultured first.
Is LPA available under NTEP?
Yes — at NTEP intermediate and national reference labs. Private NABL-accredited labs also offer it.
Should phenotypic DST still be done?
Yes — for confirmation and to test drugs the LPA does not cover (ethambutol, pyrazinamide, second-line drugs). LPA and phenotypic DST are complementary.
Related Tuberculosis / Mycobacterial tests
Tests commonly ordered alongside TB MDR SCREEN (LPA), or that help interpret an unexpected result.
Sources & references
- WHO — Line Probe Assays for TB · accessed 2026-05-30T00:00:00.000Z
- NTEP LPA Operational Guidelines · accessed 2026-05-30T00:00:00.000Z
- India TB Report 2024 · accessed 2026-05-30T00:00:00.000Z
- CDC — TB Laboratory Diagnostics · accessed 2026-05-30T00:00:00.000Z
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