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Blood Test Tracker

QuantiFERON-TB Gold Plus (IGRA)

A blood test that shows whether your immune system has reacted to TB bacteria. It cannot tell a dormant infection from active TB disease.

AI-verified against cited sources. Awaiting expert reviewAlso called QFT-Plus, QFTLOINC 71773-6

What it is

QuantiFERON-TB Gold Plus is a blood test, one type of interferon-gamma release assay (IGRA). Your blood is mixed with small pieces of tuberculosis (TB) bacteria in special tubes. If your immune cells have met TB bacteria before, they release a signal protein called interferon-gamma, and the lab measures how much. The result is printed as negative, positive or indeterminate. It is used to look for TB infection that has not made you sick, also called latent TB infection.

Why it matters

TB infection can stay quiet for years, and in some people it later becomes active TB disease. Finding it lets you and your clinician decide on follow-up and possible treatment. The test is often used for people from or with ties to countries where TB is common, healthcare workers, people about to start medicines that weaken the immune system, and close contacts of someone with TB disease. Unlike the skin test, it needs one blood draw and is not affected by the BCG vaccine. US guidance (USPSTF 2023) recommends screening adults at increased risk who have no symptoms.

If it’s high

A positive result means your immune cells reacted to TB proteins, which is a sign you may have been infected with TB bacteria at some point. It does not tell whether the infection is dormant (latent) or active, and it does not show how long ago it happened. Your clinician will usually add a symptom review and a chest X-ray. A small share of positive results are false positives, more often when the value is close to the cutoff.

Common causes

  • Latent TB infection (TB bacteria in the body but no illness)
  • Active TB disease
  • An older TB infection that is no longer active
  • Infection with certain uncommon non-TB mycobacteria that share some of the same proteins
  • A false-positive result, which is more likely when the value is near the cutoff or the test was not run correctly

What can help

  • Ask your clinician about next steps, such as a symptom review and a chest X-ray
  • Ask whether treatment for latent TB infection makes sense for you, and what the pros and cons are
  • Ask whether the people you live with or spend a lot of time with should be tested

What can affect your result

  • Immune function: a weak immune system or medicines that suppress it can lower the response and cause a false-negative or indeterminate result
  • Stage of infection: a negative result does not rule out TB infection or disease (Labcorp)
  • Sample handling: filling, mixing, temperature and delivery time of the four tubes all matter
  • Values near the cutoff can change on repeat testing, flipping from negative to positive or back
  • Test choice: a skin test or a repeat blood test is sometimes used when the first result is unclear
  • A weak immune system can lower the test's response and cause a false-negative or indeterminate result. Medicines that suppress the immune system were linked to more indeterminate results in a 2025 PLoS One study. Talk with your clinician before changing any medicine.
  • Tubes must be filled correctly, mixed and kept at room temperature. Labcorp rejects tubes that were refrigerated, frozen, centrifuged, underfilled, overfilled or older than 14 hours on arrival.
  • Heterophile antibodies or non-specific interferon-gamma from other inflammatory conditions can mask the specific response (Labcorp).
  • The BCG vaccine does not cause a positive result, because the test antigens are absent from BCG strains (CDC 2010; USPSTF 2023).

The special collection tubes must reach the lab quickly (Labcorp: within 14 hours of collection). Follow the instructions your lab gives you. The test shows your immune system's response on the day of the draw. A negative result does not rule out TB infection, and the stage of infection can affect it (Labcorp).

When to talk to a clinician

A positive result by itself is not an emergency, but talk with a clinician soon to plan follow-up. Contact a clinician promptly if you have a cough that lasts more than three weeks, coughing up blood, unexplained weight loss, fever or night sweats, because these can be signs of active TB. Call your local emergency number if you cough up a lot of blood or have trouble breathing.

Questions you could ask

  • Is a blood test or a skin test better for me?
  • Could a medicine I take or a health condition I have affect how reliable this test is?
  • If my result is indeterminate, should we repeat it or use a different test?
  • Do I need a chest X-ray or other tests to check whether TB is active?
  • Should I be treated for latent TB infection, and what are the pros and cons?
  • Should the people I live with be tested?

Questions people ask

Does a positive QuantiFERON mean I have active TB?

No. The test cannot tell latent TB infection from active TB disease. A positive result means your immune system has reacted to TB bacteria. Your clinician uses your symptoms, a chest X-ray and sometimes other tests to find out whether the infection is active.

What does indeterminate mean?

The test could not give a clear answer. This happens most often when the control that checks your immune cells' ability to respond (the mitogen) is low, or when the background (Nil) is too high. It can be caused by the sample handling or by a weakened immune response. CDC says repeating the blood test or doing a skin test might be useful if there is still a reason to test.

Can the BCG vaccine cause a positive result?

No. The proteins used in this test are not found in BCG vaccine strains or in most other mycobacteria, so a BCG vaccine in the past is not expected to cause a positive result. This is one reason the blood test is often chosen for people who had BCG.

Show the science and sources

For clinicians and researchers

QFT-Plus measures interferon-gamma (IFN-gamma) by ELISA in four tubes: Nil (background), TB1 (peptides mainly stimulating CD4+ T cells), TB2 (peptides stimulating CD4+ and CD8+ T cells) and Mitogen (positive control). Labcorp and Mayo report an interpretation plus component values (TB1 minus Nil, TB2 minus Nil, Mitogen minus Nil and Nil, in IU/mL). Interpretation follows the manufacturer's criteria: positive when TB1 minus Nil or TB2 minus Nil is at least 0.35 IU/mL and at least 25% above the Nil value; negative when both are below that and the mitogen response is adequate; indeterminate when Nil is above 8.0 IU/mL or the mitogen minus Nil value is below 0.5 IU/mL with negative TB results. Indeterminate results are most often caused by a low mitogen response. Values that sit just above or below 0.35 IU/mL often change on follow-up testing. Neither an IGRA nor a skin test can separate latent infection from active TB disease (CDC 2010), and the manufacturer describes QFT-Plus as an indirect test used with risk assessment, radiography and other evaluation. The older QuantiFERON-TB Gold In-Tube uses a single TB antigen tube with the same 0.35 IU/mL and 25% rule. USPSTF (2023) reports pooled IGRA sensitivity of roughly 81-90% (about 89% for QFT-Plus) and specificity of about 98% for QFT-Plus. Mayo and Labcorp print LOINC 71773-6 for the interpretation.

LOINC 71773-6 (primary), 71775-1. Specimen: whole-blood.

Sources

  1. QuantiFERON-TB Gold Plus, Blood (test QFT4), Mayo Clinic Laboratories (Extended Catalog)
  2. QuantiFERON-TB Gold Plus (test 182879), Labcorp
  3. QuantiFERON-TB Gold Plus (QFT-Plus), QIAGEN
  4. Updated Guidelines for Using Interferon Gamma Release Assays to Detect Mycobacterium tuberculosis Infection - United States, 2010, MMWR Recommendations and Reports (CDC) (2010)
  5. Latent Tuberculosis Infection in Adults: Screening, US Preventive Services Task Force (2023)
  6. A detailed analysis of borderline results in the QuantiFERON-TB Gold-Plus assay incorporating longitudinal follow-up: intermediate-burden setting, Microbiology Spectrum (2026)
  7. Factors associated with indeterminate QuantiFERON-TB Gold Plus Test results during the COVID-19 pandemic, PLoS One (2025)
  8. LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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