What this test measures
This test determines whether a cultured M. tuberculosis isolate is sensitive or resistant to PAS at the WHO-defined critical concentration. PAS is one of the oldest TB drugs, a bacteriostatic agent now reserved for salvage regimens in XDR-TB or extensively resistant cases where other second- and third-line drugs are exhausted.
Why it matters
PAS is rarely used in modern Indian MDR-TB practice because of its poor GI tolerability and bacteriostatic (rather than bactericidal) action. It still has a place in extreme salvage regimens — for example, post-XDR or in patients intolerant to multiple companion drugs.
Documenting PAS susceptibility is occasionally important when designing salvage regimens, especially when the team is hunting for any drug the strain remains sensitive to.
How to prepare
A positive M. tuberculosis culture 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.
PAS: where it fits
| Scenario | PAS role |
|---|---|
| Drug-sensitive TB | Not used |
| MDR-TB (modern regimens) | Rarely used |
| Pre-XDR / XDR-TB salvage | Sometimes a companion drug |
| Multi-drug-intolerant patient | Backup option |
Frequently asked questions
Is PAS used in modern TB treatment?
Rarely. WHO and NTEP regimens now rely on bedaquiline, linezolid, pretomanid, moxifloxacin and clofazimine. PAS is held in reserve for salvage scenarios.
What are the side effects?
PAS is famously poorly tolerated: nausea, vomiting, abdominal pain, diarrhoea, and rarely hepatitis, hypothyroidism (TSH monitoring), and crystalluria.
Why is PAS still tested?
For surveillance and to ensure salvage options when other drugs fail. Even small numbers of patients with XDR-TB may benefit from knowing PAS status.
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 — for selected XDR-TB patients at NTEP national reference labs.
How is PAS taken?
Usually as oral granules dissolved in juice or food, multiple times daily — adherence is challenging because of the high dose and frequency.
What if PAS is also resistant?
It significantly limits regimen design. Specialist input — including national reference centres and possibly compassionate-use access to newer agents — is essential.
Related Tuberculosis / Mycobacterial tests
Tests commonly ordered alongside ANTIBIOGRAM - MTB (PAS), 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
- India TB Report 2024 · accessed 2026-05-30T00:00:00.000Z
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