Lab result explainedInflammationLast updated November 10, 2026

TB blood test (QuantiFERON and T-SPOT): what positive, negative and indeterminate results mean

TB blood test (interferon-gamma release assay) (IGRA) · Tuberculosis screening (latent TB infection testing) · reported in IU/mL of interferon-gamma (QuantiFERON); spot-forming cells per well (T-SPOT.TB)

A TB blood test, formally an interferon-gamma release assay (IGRA), checks whether your immune system has met Mycobacterium tuberculosis, the bacterium that causes tuberculosis. The two tests used in the US are QuantiFERON-TB Gold Plus and T-SPOT.TB. Your blood is mixed in the laboratory with proteins found only in TB bacteria; if your T cells have been primed by a previous TB infection they release interferon-gamma, and the test measures how much. On QuantiFERON, a result of 0.35 IU/mL or more above the control (nil) tube is positive. On T-SPOT, 8 or more spots is positive and 5 to 7 is borderline.

A positive result means TB infection, but in the great majority of people it means latent infection: live bacteria walled off by the immune system, no symptoms and no risk of passing it on. Up to 13 million people in the US have latent TB. Without treatment roughly 5 to 10 percent of them will develop active TB disease at some point, about half within the first two years after infection, and far more if the immune system is weakened. The blood test cannot tell latent from active disease, so a positive result is always followed by a symptom review and a chest X-ray, and sometimes sputum tests.

The blood test has replaced the tuberculin skin test for most adults because it needs one visit, is read by a machine rather than a ruler, and is not affected by the BCG vaccine that many people born outside the US received as children. Its weak points are a window of up to eight weeks after exposure during which it can still be negative, an indeterminate rate of about 2 to 5 percent, and low-positive values just above the cut-off that often turn negative when repeated.

IGRA at a glance

What it is: a blood test that measures interferon-gamma released by your T cells when they meet TB-specific antigens; QuantiFERON-TB Gold Plus and T-SPOT.TB are the two US tests.
Positive (QuantiFERON): TB antigen tube minus nil tube is 0.35 IU/mL or higher and at least 25 percent above nil. Positive (T-SPOT): 8 or more spots; 5 to 7 is borderline and is repeated.
Negative: below those cut-offs with a working mitogen (positive control) response. Indeterminate: a control tube failed, usually because of immune suppression or sample handling; it says nothing about TB and is repeated.
A positive test means TB infection, almost always latent; it does not mean active or contagious TB. A chest X-ray and symptom check decide which.
BCG vaccination does not cause a positive IGRA, which is the main reason it is preferred over the skin test. Rare cross-reaction with M. kansasii, M. marinum and M. szulgai.
Next step after a positive: chest X-ray; if it is clear and you have no symptoms, a 3 to 4 month course of preventive treatment is usually recommended to stop latent infection becoming active disease.

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What IGRA measures

Both tests rely on two proteins, ESAT-6 and CFP-10, that are encoded in a region of the TB genome (RD1) missing from the BCG vaccine strain and from almost all environmental mycobacteria. T cells that have previously encountered TB recognize these proteins and release interferon-gamma within hours. The amount released is a readout of whether TB-specific memory T cells exist, which is what infection leaves behind, not of how many bacteria are present or whether they are active.

QuantiFERON-TB Gold Plus uses four tubes: nil (no stimulus), TB1 (antigens that stimulate CD4 T cells), TB2 (antigens that also stimulate CD8 T cells) and mitogen (a general T-cell stimulant that proves the cells are alive and responsive). Interferon-gamma is measured by ELISA in IU/mL and the nil value is subtracted from the others. T-SPOT.TB isolates white cells from the blood, places a fixed number (250,000) in wells with the same antigens, and counts the individual cells that release interferon-gamma as spots. The nil well must have 10 spots or fewer. A positive-control well below 20 spots makes the test invalid only when both antigen panels (minus nil) are 4 spots or fewer; a positive or borderline panel still counts.

In people with culture-proven active TB, QuantiFERON is positive in about 80 percent and T-SPOT in about 90 percent, so a negative result does not exclude TB disease. Specificity is 95 to 99 percent in people without TB. The tubes must be shaken well and incubated within 16 hours of the draw (T-SPOT samples processed within 8 hours, or 32 hours with a stabilizer), and results take one to three days. A positive IGRA is not a strong predictor of who will go on to develop TB: in pooled studies about 2 to 3 percent of untreated IGRA-positive people progressed over two years, higher in recent contacts and the immunosuppressed.

Normal range for IGRA

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (IU/mL of interferon-gamma (QuantiFERON); spot-forming cells per well (T-SPOT.TB))	Notes
QuantiFERON negative	TB1 minus nil and TB2 minus nil both below 0.35 IU/mL (or below 25 percent of nil), with mitogen minus nil 0.5 IU/mL or more	TB infection unlikely, provided the exposure was more than 8 weeks ago and the immune system is intact.
QuantiFERON positive	TB1 minus nil or TB2 minus nil 0.35 IU/mL or more, and at least 25 percent of the nil value	TB infection likely. Does not distinguish latent from active disease.
QuantiFERON low positive (unofficial 'borderline' zone)	0.35–0.99 IU/mL	Not an official category, but values in this band revert to negative on repeat in roughly half of low-risk people. Many programs repeat the test before treating.
QuantiFERON indeterminate	Nil above 8.0 IU/mL, or mitogen minus nil below 0.5 IU/mL with negative TB tubes	The controls failed; the result says nothing about TB. Causes: immunosuppression, low lymphocyte count, delayed or mishandled sample. Repeat.
T-SPOT.TB negative	4 spots or fewer (panel A or B minus nil)	TB infection unlikely, same caveats as above.
T-SPOT.TB borderline	5, 6 or 7 spots	US-specific category; retest, ideally with a fresh sample.
T-SPOT.TB positive	8 spots or more	TB infection likely.
T-SPOT.TB invalid	Nil above 10 spots, or positive control below 20 spots when both antigen panels minus nil are 4 spots or fewer	Equivalent of indeterminate; repeat. A positive or borderline panel is still reported even if the positive control is low.
Tuberculin skin test, for comparison	Positive at 5 mm (HIV, immunosuppressed, recent contact, old TB on X-ray), 10 mm (recent arrival from a high-prevalence country, health-care worker, children under 4, congregate settings) or 15 mm (no risk factors)	Read 48 to 72 hours after placement; BCG and environmental mycobacteria can cause false positives.

Positive TB blood test

A positive IGRA means your T cells recognize TB-specific proteins, which almost always means you were infected with TB at some point. For most people reading this, that is latent TB infection: the bacteria are alive but contained, you have no symptoms, your chest X-ray is normal and you cannot infect anyone. Latent TB is common in people born or who lived for years in countries where TB is widespread, in household contacts of someone with active TB, in health-care and correctional workers, and in older adults who were infected decades ago.

The test cannot tell latent infection from active disease, and it stays positive after treatment, usually for years. So the next step is always the same: a review of symptoms (cough lasting more than two to three weeks, fever, night sweats, weight loss, coughing blood) and a chest X-ray. If both are clear, you have latent TB and preventive treatment is offered, because it cuts the lifetime risk of active disease by 60 to 90 percent. If either raises concern, sputum samples are sent for smear, culture and a molecular test before any decision is made.

How much weight the result carries depends on your risk. In a recent contact of a TB case or someone from a high-prevalence country, a positive IGRA is almost always true. In a US-born person with no exposure history and a value just over 0.35 IU/mL, a false positive from assay variability is a real possibility and a repeat test is reasonable before treatment. A positive result in someone about to start a TNF inhibitor, chemotherapy, dialysis or a transplant is taken seriously regardless of the number, because those treatments sharply raise the risk of reactivation.

Common causes

Latent TB infection
By far the most common reason. Infection occurred months to decades ago and is contained; a quarter of the world's population and up to 13 million people in the US carry it. The risk of it becoming active is highest in the first two years after infection and with HIV, diabetes, kidney failure, immunosuppressive drugs, smoking and undernutrition.
Active TB disease
A positive IGRA with cough, fever, night sweats, weight loss or an abnormal chest X-ray is investigated as active TB until proven otherwise. Note that 10 to 20 percent of people with active TB have a negative IGRA, so the test is not used on its own to rule disease in or out.
Previously treated TB
Memory T cells persist, so people who completed treatment for latent or active TB years ago usually remain positive. Repeating the IGRA after treatment is not useful and is not recommended.
Recent exposure (conversion)
A test that was negative and is now positive within two years, particularly after contact with an infectious case, is a conversion and carries the highest risk of progression; preventive treatment is strongly recommended.
Cross-reaction with a few non-tuberculous mycobacteria
M. kansasii, M. marinum and M. szulgai share the ESAT-6 and CFP-10 antigens and can produce a positive IGRA; M. avium complex and BCG do not.
Low-positive value from test variability
The assay has day-to-day variation of around 0.2 to 0.5 IU/mL near the cut-off. In serial testing of US health-care workers, most 'conversions' were below 1.0 IU/mL and half or more reverted on retesting, which is why low positives in low-risk people are repeated.
Pre-analytical problems
Overfilled or under-mixed tubes and delayed incubation change interferon-gamma values; a mislabelled sample is rare but possible, and an unexpected positive in a low-risk person is a reason to redraw.
Boosting after a skin test
Placing a tuberculin skin test can increase the IGRA response if blood is drawn more than three days later; if both tests are needed, the blood is drawn first or on the same day the skin test is placed.

Symptoms that can go with it

None at all with latent TB infection, which is the usual situation
Cough lasting more than 2 to 3 weeks, with or without sputum, if TB is active
Coughing up blood or blood-streaked sputum
Fever, drenching night sweats and unexplained weight loss
Loss of appetite, fatigue and chest pain when breathing or coughing
Painless swollen lymph nodes in the neck, back pain, or joint swelling with TB outside the lungs
Headache, confusion or neck stiffness with TB meningitis, which is an emergency

Negative or indeterminate TB blood test

A negative result means no TB-specific T-cell response was detected and the control tube worked. For someone with a normal immune system tested more than eight weeks after any possible exposure, that makes TB infection unlikely, and in low-risk people the negative predictive value is above 99 percent. Most pre-employment, immigration and pre-treatment screens end here.

A negative test does not fully exclude TB. The immune response takes up to eight weeks to develop after exposure, so contacts of an infectious case are retested 8 to 10 weeks after their last exposure. Sensitivity in active disease is only 80 to 90 percent and lower still in people with HIV and a low CD4 count, in those on high-dose steroids, TNF inhibitors or chemotherapy, in children under 2 and in people over 80. Anyone with symptoms of TB is investigated with a chest X-ray and sputum tests whatever the blood test shows.

An indeterminate result is a failed test, not a sign of infection. It usually means the mitogen (positive control) tube did not respond, which happens when lymphocytes are few or suppressed, or the nil tube was too high because of background immune activation. It is repeated, and if it is indeterminate twice a skin test or a different IGRA can be used, with the clinical picture deciding what happens next.

Common causes

No TB infection
The expected explanation for most people, especially those born in the US with no known contact with TB.
Tested in the window period
T-cell responses take 2 to 8 weeks to develop. A test done within 8 weeks of exposure can be falsely negative; contacts are retested 8 to 10 weeks after the last exposure.
Immunosuppression
HIV with a CD4 count under 200, prednisone 15 mg a day or more for a month, TNF inhibitors, chemotherapy, transplant medicines, dialysis and severe malnutrition all reduce interferon-gamma production and cause both false negatives and indeterminate results.
Active TB with an exhausted immune response
Severe or disseminated TB, TB meningitis and TB in people with HIV can produce a negative or indeterminate result; the test is never used to rule out active disease.
Very young or very old age
Children under 2 and adults over 80 have weaker responses; most guidelines prefer the skin test under age 2 and accept either test from age 2 (some from age 5).
Low lymphocyte count
Lymphopenia from any cause (chemotherapy, viral infection, lupus, sepsis) leaves too few T cells to respond and is the commonest reason for an indeterminate QuantiFERON.
Sample handling problems
Tubes not shaken ten times, under-filled tubes, blood left too long before incubation, or temperature extremes during transport lower the mitogen response and raise the indeterminate rate from 2 percent to 10 percent or more.
Recent live-virus vaccination
Measles, MMR, varicella and yellow fever vaccines can suppress T-cell responses for several weeks; the CDC advises drawing the IGRA on the same day as the vaccine or waiting 4 to 6 weeks.

Symptoms that can go with it

None; a negative result causes no symptoms
If you have a cough over 3 weeks, fever, night sweats or weight loss despite a negative test, you still need a chest X-ray and sputum tests, because the test misses 10 to 20 percent of active TB

Read it alongside

Chest X-ray — the essential next step after any positive IGRA; a normal film with no symptoms defines latent TB, while cavities, upper-lobe shadowing or enlarged lymph nodes point to active disease.
Sputum AFB smear, culture and NAAT (such as Xpert MTB/RIF) — the tests that actually diagnose active TB; three sputum samples are standard when there is a cough or an abnormal X-ray.
HIV test — recommended for everyone with a positive TB test, because HIV multiplies the risk of progression and changes treatment.
Lymphocyte count — a low count explains many indeterminate results and signals that a negative result is less reliable.
ALT and AST — measured before and during preventive treatment, since isoniazid and rifampin can injure the liver; a raised baseline changes the regimen chosen.
Hepatitis B and C serology — checked before treatment in people with risk factors, because viral hepatitis raises the risk of drug-induced liver injury.
CRP and ESR — usually normal in latent TB; raised values with symptoms support active disease but are not specific.
Tuberculin skin test — occasionally used alongside the IGRA in children, in the immunosuppressed, or to resolve a borderline result.

What usually happens next

After a positive test, expect a symptom review and a chest X-ray within days; if both are clear you will be offered preventive treatment, most often 12 weekly doses of isoniazid plus rifapentine (3HP) or 4 months of daily rifampin (4R), which are now preferred over 9 months of isoniazid.
If the value is low positive (under about 1.0 IU/mL) and you have no risk factors, ask whether a repeat test is appropriate before treatment; conversions in this band often revert.
After an indeterminate result, the test is simply repeated with careful sample handling; if it is indeterminate again, a skin test or the other type of IGRA may be used, and the decision rests on your risk and chest X-ray.
If you were exposed to someone with infectious TB and your test is negative, a second test 8 to 10 weeks after the last contact is standard; children under 5 and immunosuppressed contacts are sometimes started on preventive treatment while waiting.
Keep a copy of the result. A positive IGRA stays positive after treatment, so documentation of completed treatment is what future employers and clinics need, not a repeat test.

Diet, lifestyle and natural levers with evidence

What can move IGRA once the cause is known. Nothing here replaces finding the cause.

Get the timing right after an exposure

Strong evidence

Nothing you eat or take changes an IGRA result, but timing does. T-cell responses take 2 to 8 weeks to appear after infection, so a test done in the first weeks after contact with an infectious case can be falsely negative. CDC contact-investigation guidance is to test contacts promptly and, if negative, again 8 to 10 weeks after the last exposure. If you need both a skin test and a blood test, have the blood drawn before or on the same day the skin test is placed, because tuberculin can boost the IGRA if blood is taken days later.

Know which medicines and conditions blunt the test

Moderate evidence

Prednisone at 15 mg a day or more, TNF inhibitors, methotrexate at higher doses, chemotherapy, uncontrolled HIV and a lymphocyte count under 1,000/µL each raise the indeterminate rate from about 2 percent to 10 to 20 percent and increase false negatives. Where possible the test is drawn before these treatments start; for a screen before a biologic, that is the standard sequence. Live-virus vaccines (MMR, varicella, yellow fever) can suppress the response for 4 to 6 weeks, so the blood is drawn the same day as the vaccine or afterwards.

Do not stop prescribed immunosuppressants to improve a TB test result; tell the clinician ordering the test what you take so the result is interpreted correctly.

If you have latent TB, lower the factors that drive progression

Preliminary evidence

Preventive antibiotics are the only intervention proven to stop latent TB becoming active, cutting the risk by 60 to 90 percent. Observational data show that smoking roughly doubles the risk of developing TB disease, poorly controlled diabetes about triples it and heavy alcohol use raises it two- to threefold, so quitting smoking, limiting alcohol and keeping A1C in range are sensible alongside treatment. Vitamin D has been tested properly: a trial of 8,851 Mongolian children given 14,000 IU weekly for three years found no reduction in new TB infection or disease, so supplements are not a substitute for treatment.

Do not take herbal or supplement products marketed for TB in place of, or alongside, preventive treatment without checking for liver interactions; isoniazid and rifampin already stress the liver.

When to see a clinician promptly

Coughing up blood, breathlessness, chest pain or a cough lasting more than 3 weeks after any TB test result: same-week assessment for active TB, which is infectious and treatable.
Fever, drenching night sweats and unexplained weight loss, especially with a positive IGRA or after known TB contact.
Severe headache, neck stiffness, confusion or drowsiness in anyone with a positive TB test: possible TB meningitis, needs emergency care.
A positive result if you have HIV, are on a TNF inhibitor or other immunosuppressant, are pregnant, have diabetes or kidney failure, or are a close contact of someone with active TB; these groups need prompt evaluation and usually treatment.
A repeat indeterminate result or a borderline T-SPOT when you are about to start a biologic, chemotherapy or transplant work-up.
During preventive treatment: nausea, vomiting, dark urine, yellow eyes, abdominal pain or unusual fatigue, which can signal drug-induced liver injury and need a same-day call.

Frequently asked questions

What does a positive QuantiFERON test mean?

It means your T cells released 0.35 IU/mL or more of interferon-gamma in response to TB proteins, which almost always means you have been infected with TB at some point. In the great majority of people this is latent infection: no symptoms, a normal chest X-ray and no risk to others. A positive result leads to a symptom check and chest X-ray to exclude active disease, and then usually to a short course of preventive treatment.

What does an indeterminate QuantiFERON result mean?

The test failed its internal controls, so it cannot say whether you have TB infection or not. It happens in about 2 to 5 percent of tests, more often in people on steroids or immunosuppressants, with HIV, with a low lymphocyte count, in young children and the elderly, or when the blood tubes were handled badly. The usual response is to repeat the test, and if that is also indeterminate, to use a skin test or the other IGRA type and rely on the chest X-ray and your risk factors.

Is a TB blood test better than a skin test?

For most adults, yes. It needs one visit instead of two, is not affected by the BCG vaccine, has fewer false positives from environmental mycobacteria, and gives an objective number. The skin test is still preferred for children under 2 and is acceptable from age 2 to 5, and it is cheaper. Neither test can tell latent infection from active disease, and neither is reliable enough to rule out active TB in someone with symptoms.

Can a TB blood test be a false positive?

Yes, though uncommonly. Specificity is about 95 to 99 percent, so in low-risk populations a meaningful share of positives, especially values between 0.35 and 1.0 IU/mL, are false and revert on repeat testing. Infection with M. kansasii, M. marinum or M. szulgai, test-to-test variability near the cut-off, a skin test placed more than three days before the blood draw, and laboratory handling errors are the known causes. BCG vaccination does not cause a false-positive IGRA.

How long after exposure does a TB blood test turn positive?

Usually within 2 to 8 weeks. A test done in the first few weeks after contact with someone who has infectious TB can be negative even though infection has occurred, so contacts with a negative first test are retested 8 to 10 weeks after their last exposure. A test that becomes positive within two years of a previous negative is a conversion and carries the highest risk of progressing to active TB.

Does the BCG vaccine affect the QuantiFERON or T-SPOT test?

No. The antigens used (ESAT-6 and CFP-10) are missing from the BCG strain, so people vaccinated as children test negative unless they have actually been infected with TB. This is the main reason IGRAs are preferred over the skin test in people born in countries that use BCG, where skin-test false positives are common.

Will my TB blood test stay positive after treatment?

Usually yes, often for many years, because the memory T cells that drive the test persist after the bacteria are gone. Repeating the test after treatment is not recommended and does not prove that treatment worked. Keep documentation that you completed treatment; that is what future screening programs will ask for.

Do I need to fast or prepare for a TB blood test?

No fasting is needed and there are no diet restrictions. Tell the clinic about steroids, biologics, chemotherapy, HIV or a recent live-virus vaccine, all of which can cause an indeterminate or false-negative result, and mention any skin test placed in the past few days. The sample is drawn into special tubes and must reach the lab within hours, so draws are sometimes limited to certain days or times.

References

Testing for tuberculosis infection: interferon-gamma release assays (IGRAs). Centers for Disease Control and Prevention (2024)
Tuberculosis (TB) screening. MedlinePlus, US National Library of Medicine (2023)View source
Mazurek GH, Jereb J, Vernon A, et al.. Updated guidelines for using interferon gamma release assays to detect Mycobacterium tuberculosis infection, United States, 2010. MMWR Recommendations and Reports (2010)
Lewinsohn DM, Leonard MK, LoBue PA, et al.. Official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention clinical practice guidelines: diagnosis of tuberculosis in adults and children. Clinical Infectious Diseases (2017)
Pai M, Denkinger CM, Kik SV, et al.. Gamma interferon release assays for detection of Mycobacterium tuberculosis infection. Clinical Microbiology Reviews (2014)
Sterling TR, Njie G, Zenner D, et al.. Guidelines for the treatment of latent tuberculosis infection: recommendations from the National Tuberculosis Controllers Association and CDC, 2020. MMWR Recommendations and Reports (2020)
US Preventive Services Task Force. Screening for latent tuberculosis infection in adults: US Preventive Services Task Force recommendation statement. JAMA (2023)
Slater ML, Welland G, Pai M, Parsonnet J, Banaei N. Challenges with QuantiFERON-TB Gold assay for large-scale, routine screening of U.S. healthcare workers. American Journal of Respiratory and Critical Care Medicine (2013)
Metcalfe JZ, Cattamanchi A, McCulloch CE, Lew JD, Ha NP, Graviss EA. Test variability of the QuantiFERON-TB Gold In-Tube assay in clinical practice. American Journal of Respiratory and Critical Care Medicine (2013)
Ganmaa D, Uyanga B, Zhou X, et al.. Vitamin D supplements for prevention of tuberculosis infection and disease. New England Journal of Medicine (2020)
Tuberculosis: diagnosis and treatment. Mayo Clinic (2023)View source
Tuberculosis (TB). Merck Manual Professional Version (2024)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.