What this test measures
IGRAs measure the release of interferon-gamma (IFN-γ) from T cells stimulated by Mycobacterium tuberculosis-specific antigens (ESAT-6 and CFP-10). Two main commercial assays are used: QuantiFERON-TB Gold (and Gold Plus) which uses whole blood and measures IFN-γ by ELISA, and T-SPOT.TB which uses isolated mononuclear cells and counts IFN-γ-producing cells.
Unlike the tuberculin skin test (TST / Mantoux), IGRAs use antigens absent from BCG and most environmental mycobacteria — so they are not affected by prior BCG vaccination (universal in India). They detect immune sensitisation to TB but do not distinguish active from latent infection.
Why it matters
India has the world's largest population living with latent TB infection — an estimated 35–40% of the adult population have immune evidence of past exposure. NTEP's TB Preventive Treatment (TPT) programme aims to treat high-risk individuals before they progress to active disease. IGRAs are used to screen for latent TB in:
- Healthcare workers and laboratory staff (pre-employment and annual screening). - People living with HIV (TB is the leading cause of HIV-related death). - Household contacts of confirmed TB cases. - Patients before starting biologics (anti-TNF, JAK inhibitors), transplant, dialysis, or other immunosuppression. - Migrants and travellers from high-burden countries.
A positive IGRA in any of these scenarios usually triggers TPT — typically a 3-month rifapentine + isoniazid (3HP) or 6-month isoniazid regimen.
How to prepare
No fasting required. A simple blood sample (whole blood) is collected into specialised tubes (3 or 4 depending on the kit) for QuantiFERON, or a heparinised sample for T-SPOT. Avoid major immunosuppressive medications in the days before testing if possible — they can blunt the immune response and cause false negatives.
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 |
|---|---|---|---|
| TB antigen IFN-γ response (IU/mL)[1][2] | < 0.35 IU/mL above nil = negative | Negative — no immune evidence of TB exposure. Does not exclude very recent infection (within 8 weeks) or active TB in advanced immunosuppression. Repeat in 8 weeks if recent exposure is suspected. | ≥ 0.35 IU/mL above nil = positive — immune evidence of past M. tuberculosis exposure. Distinguish active TB from latent TB clinically (symptoms, chest X-ray, sputum); start TPT for latent TB in eligible individuals. |
| Mitogen control (IU/mL) | ≥ 0.5 IU/mL (positive control) | Failed mitogen control — immune response too weak to interpret the test. Indeterminate result. Common in severely immunosuppressed patients. | — |
IGRA vs Tuberculin Skin Test (TST / Mantoux)
| Feature | IGRA (this test) | TST / Mantoux |
|---|---|---|
| Affected by BCG vaccine | No | Yes (false positives) |
| Number of visits | 1 (blood draw) | 2 (placement + reading) |
| Detects latent TB | Yes | Yes |
| Distinguishes active from latent | No | No |
| Use in young children (<5 y) | Lower data; TST often preferred | Standard |
| Use in immunosuppression | Better — fewer false negatives | Can be falsely negative |
Frequently asked questions
What is the difference between IGRA and TB Gold?
IGRA is the general term; "TB Gold" is the brand name (QuantiFERON-TB Gold Plus is the current version). Both refer to interferon-gamma release assays for latent TB.
Does my BCG vaccine affect the result?
No — IGRAs use TB-specific antigens absent from BCG, so prior BCG vaccination (universal in India) does not cause false positives.
Can IGRA tell me if I have active TB?
No. A positive IGRA confirms past or present M. tuberculosis exposure but cannot distinguish active from latent infection. Symptoms, chest X-ray and microbiological tests (CBNAAT, smear, culture) are needed to diagnose active TB.
What happens if my IGRA is positive?
Your doctor will rule out active TB (with symptoms review, chest X-ray, and CBNAAT if any symptoms). If active TB is excluded, TB preventive treatment (TPT) is offered — typically 3HP (3 months rifapentine + isoniazid weekly) or 6H (isoniazid daily for 6 months).
Is IGRA available under NTEP?
IGRA is offered in private healthcare and some tertiary public hospitals; the tuberculin skin test (TST) is more widely available in NTEP settings. NTEP's TPT pathway accepts either test.
How long does the result take?
About 24–48 hours from blood collection. The blood needs to be incubated for 16–24 hours before the IFN-γ measurement.
Can IGRA be falsely negative?
Yes — in advanced immunosuppression (low CD4 HIV, severe malnutrition, on biologics), the IFN-γ response can be blunted. The mitogen control flag (indeterminate result) usually catches these cases.
Related Tuberculosis / Mycobacterial tests
Tests commonly ordered alongside IGRAS (Gamma Interferon) / TB Gold, or that help interpret an unexpected result.
Sources & references
- CDC — Tuberculosis Testing · accessed 2026-05-30T00:00:00.000Z
- WHO — Latent TB Infection Guidelines · accessed 2026-05-30T00:00:00.000Z
- NTEP TB Preventive Treatment Guidelines · accessed 2026-05-30T00:00:00.000Z
- India TB Report 2024 · accessed 2026-05-30T00:00:00.000Z
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