QuantiFERON-TB Gold Plus: What the Test Is, How to Read the Result, and What It Costs

QuantiFERON-TB Gold Plus — written QFT-Plus, and usually just "QuantiFERON" — is a laboratory blood test for TB infection, made by QIAGEN. It belongs to a class of tests called interferon-gamma release assays (IGRAs); the other IGRA sold in the US is T-SPOT.TB. Both do the same job the century-old tuberculin skin test does, in one visit and without the skin test's known weaknesses.

The principle is simple. If your immune system has ever encountered Mycobacterium tuberculosis, some of your T cells remember it. In the lab, a sample of your blood is mixed with two proteins that exist in the TB bacterium but not in the BCG vaccine strain or in most environmental mycobacteria (ESAT-6 and CFP-10). T cells that recognise those proteins release a signalling molecule, interferon-gamma, and the assay measures how much was released. A meaningful release means your immune system has been primed by the bacterium — that is, TB infection.

What the test does not do matters just as much: it cannot tell whether that infection is dormant (latent TB infection, which is not contagious and causes no symptoms) or has become active TB disease. That distinction is made clinically — by symptoms, a chest X-ray and, when needed, sputum tests — after a positive result.

QFT-Plus is run in four tubes, each holding a portion of the same blood sample:

• Nil — blood with nothing added. It measures the interferon-gamma your cells release on their own, the background that is subtracted from everything else.
• TB1 — blood with TB-specific peptides designed to stimulate CD4 "helper" T cells.
• TB2 — blood with peptides that stimulate CD4 and also CD8 "cytotoxic" T cells, the addition that makes this the "Plus" version; CD8 responses are thought to be more prominent in recent infection and in active disease.
• Mitogen — blood with a general immune stimulant. It is the positive control: if your cells cannot respond even to this, the test cannot be interpreted.

The tubes are incubated at body temperature for 16–24 hours, then the plasma is tested for interferon-gamma. The result is calculated as TB1 minus Nil and TB2 minus Nil, in international units per millilitre (IU/mL).

At Quest and Labcorp the draw itself is one tube — the laboratory transfers the blood into the four QFT tubes within 16 hours of collection. That handling window is why some collection sites restrict QuantiFERON draws to certain days or morning hours, and why a weekday morning draw gives your sample the best chance of a clean result.

The tuberculin skin test (TST, or PPD) requires two visits — an injection, then a return 48–72 hours later to have the reaction measured by a trained reader — and it reads falsely positive in many people who received the BCG vaccine in childhood, which is most of the world outside the United States. QuantiFERON's TB-specific proteins are absent from BCG, so prior vaccination does not affect it, and it is read by a machine rather than a ruler.

The CDC recommends an IGRA over the skin test for people who have received BCG and for anyone unlikely to return for a skin-test reading, and most US universities, healthcare employers and volunteer programmes now accept or prefer it. US immigration medical examinations (Form I-693) require an IGRA for applicants aged two and older. The skin test is still used for children under two and in some programmes that perform serial testing; our comparison of the two tests covers those cases.

Your report will show one of three interpretations, together with the underlying numbers.

Negative. The TB antigen tubes released little or no interferon-gamma above the Nil background (TB1−Nil and TB2−Nil both below 0.35 IU/mL, or below 25% of the Nil value), and the Mitogen tube responded normally. TB infection is unlikely. Two caveats come with every negative result: if your exposure to TB was within the last 8–10 weeks, your immune system may not have had time to respond and the test should be repeated after that window; and if you have symptoms — a cough lasting three weeks or more, coughing up blood, fever, night sweats, unexplained weight loss — a negative blood test does not exclude TB disease and you should be evaluated by a clinician.

Positive. One or both TB antigen tubes released interferon-gamma at or above 0.35 IU/mL and at least 25% above the Nil value. This means your immune system has been primed by the TB bacterium: TB infection is likely. It does not say when you were infected or whether the infection is active. The next steps are a clinical evaluation, a chest X-ray and a symptom review; if active disease is excluded, treatment for latent TB infection is usually offered, because it sharply lowers the lifetime risk of the infection progressing. A positive IGRA is not repeated to "check" it — repeat testing does not change the interpretation.

Indeterminate. The test could not be interpreted, for one of two reasons: the Nil tube's background was too high (above 8.0 IU/mL), or the Mitogen tube failed to respond (below 0.5 IU/mL above Nil). Indeterminate results are more common in people with weakened immune systems, during or just after an acute illness, in very young children, and when the sample was handled late or incorrectly. An indeterminate result is neither positive nor negative; it is usually repeated, sometimes with the other IGRA (T-SPOT.TB) or a skin test.

Laboratories report the raw values (Nil, TB1−Nil, TB2−Nil, Mitogen−Nil) alongside the interpretation. Values just over the 0.35 cut-off, especially in someone at low risk, are sometimes repeated before treatment decisions, because results in that narrow zone can fluctuate.

In people with culture-confirmed TB disease, IGRAs including QFT-Plus are positive roughly 80–90% of the time — their sensitivity is similar to the skin test's, and no test for TB infection reaches 100%. Their advantage is specificity: in low-risk, BCG-vaccinated populations QFT-Plus is specific in more than 97% of cases, where the skin test produces many false positives. Accuracy is lower in young children and in people whose immune systems are suppressed (HIV with a low CD4 count, chemotherapy, biologic drugs, high-dose steroids), where indeterminate and false-negative results are more common.

Because the test measures an immune response rather than the bacterium itself, it stays positive after treatment of TB infection and after past disease — it cannot be used to confirm that treatment worked, and a known positive person does not need retesting for school or work.

Ordered through TestWell, the QuantiFERON-TB Gold Plus test is $91.99 plus one $6 physician-authorization fee — $97.99 all-in, with no insurance involved. A licensed physician reviews and authorizes the order (usually within minutes), you walk into any Quest Diagnostics or Labcorp patient service center with the requisition we email you (BioReference in New York and New Jersey), and the result posts to your account in 3–5 business days with the four tube values and the interpretation. The report can be downloaded for a school, employer or clinician.

The same test is listed at $149 on the laboratories' own consumer sites (Quest Health and Labcorp OnDemand) and $130–200 at most other online lab services; through a doctor's office or urgent care it is a visit fee plus the lab charge, commonly $75–350. Our TB test cost guide compares every route, including health departments, which often run the skin test free or for a small fee.

No fasting is needed. Vaccines given in the previous four to six weeks (MMR, varicella, yellow fever and other live vaccines) can suppress the response; either have the blood drawn the same day as the vaccine or wait four weeks. Go on a weekday, ideally Monday to Thursday in the morning, so the sample reaches the laboratory within its handling window.

Programmes that ask for a TB test are asking for evidence of screening for TB infection, and most now specify or accept an IGRA. Check the exact wording of your requirement before ordering: some ask for the result to be dated within a set number of months, some ask for the laboratory report rather than a summary, and immigration medical examinations must be ordered by the civil surgeon performing the exam rather than brought to it. Our guides to TB tests for university enrollment and for employment cover the common requirement types and what each accepts.

If your result is positive, the programme will usually ask for a chest X-ray and a clinician's evaluation; that evaluation, not the blood test, is what determines next steps. If you have any of the symptoms listed above, see a clinician first regardless of what a form requires.