TB Blood Test (IGRA): What Positive, Negative & Indeterminate Mean
Also known as: IGRA, QuantiFERON-TB Gold, interferon-gamma release assay, T-SPOT.TB, TB blood test
A TB blood test — an interferon-gamma release assay (IGRA), such as QuantiFERON-TB Gold or T-SPOT.TB — checks whether your immune system reacts to tuberculosis bacteria, indicating TB infection. A normal result is 'negative' (no infection detected); a 'positive' result means you've been infected with TB, but it does not tell you whether the infection is latent (dormant) or active disease. Results can also be 'indeterminate,' which usually means the test should be repeated.
TB Blood Test normal range
| Category | Range (positive / negative / indeterminate) |
|---|---|
| Negative (normal) | No TB infection detectedImmune response below the assay cutoff |
| Positive | TB infection detectedLatent vs. active disease must be determined clinically |
| Indeterminate / borderline | InconclusiveUsually repeated; can occur with immune suppression |
| Interpretation varies by assay | Use your own reportQuantiFERON and T-SPOT use different measures |
Reference ranges vary by laboratory. Use the range printed on your own report as the definitive comparison.
What high TB Blood Test can mean
- A positive result indicates infection with tuberculosis bacteria
- Latent TB infection (dormant, not contagious) — the most common meaning of a positive test
- Active TB disease (when symptoms and imaging support it) — needs prompt care
- Prior TB exposure; IGRAs are generally not affected by the BCG vaccine
What low TB Blood Test can mean
- A negative result means no TB infection was detected and is the normal finding
- Very early infection or a weakened immune system can occasionally cause a false-negative, so context matters
What to do about an abnormal result
- A positive TB blood test means infection, not necessarily active disease — a chest X-ray, symptom review, and sometimes sputum testing determine which.
- Compare your result to your own report, and share it with your clinician, especially if you have symptoms like cough, fever, night sweats, or weight loss.
- An indeterminate result is usually repeated; it can happen with immune-suppressing conditions or medications.
- Don't self-diagnose from one result — latent TB is treatable to prevent progression, and management is guided by a clinician.
Understand your own results
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Frequently asked questions
What does a positive TB blood test mean?
A positive IGRA (TB blood test) means you've been infected with tuberculosis bacteria. Most positives reflect latent TB — a dormant, non-contagious infection — rather than active disease. Determining which requires a chest X-ray, a symptom review, and sometimes additional testing.
Is the TB blood test better than the skin test?
The IGRA blood test has a key advantage: it isn't affected by prior BCG vaccination, so it produces fewer false positives in vaccinated people, and it needs only one visit. Both tests detect infection rather than distinguishing latent from active TB, so a positive result still requires follow-up.
What does an indeterminate TB test result mean?
An indeterminate (or borderline) result means the test couldn't be clearly interpreted, often due to sample handling or a weakened immune system. It doesn't mean you do or don't have TB — it usually means the test should be repeated, sometimes with additional evaluation.
Related biomarkers
Medically reviewed by Antonieta Rueda, MD and Kyle R. Toth, MD · Last reviewed July 6, 2026
This article is for informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or your lab results.