T-SPOT.TB (IGRA result)
Counts white blood cells that react to tuberculosis proteins. It shows whether your immune system has met TB bacteria, not whether you are sick.
AI-verified against cited sources. Awaiting expert reviewAlso called T-SPOT, TSPOT, T-Spot.TBLOINC 74281-7
What it is
T-SPOT.TB is a blood test for tuberculosis (TB) infection, known as an interferon-gamma release assay or IGRA. The lab separates white blood cells from your blood and exposes them to two proteins made by the TB bacterium, ESAT-6 and CFP-10. It then counts the cells that react, shown as spots. Those proteins are not in any BCG vaccine strain and are missing from most other mycobacteria, so a past BCG vaccination should not cause a positive result. The result is reported as positive, negative, borderline or invalid.
Why it matters
Clinicians use TB blood tests to find people who carry TB bacteria without symptoms, called latent TB infection, for example before certain treatments or for work in health care. A positive result means your immune system has been exposed to TB. It does not mean you have active TB disease, and the test cannot tell the two apart. Your clinician usually looks at your symptoms and may order a chest X-ray. Finding latent infection matters because it can be treated to lower the chance it becomes active disease later.
If it’s low
A negative result means the test did not find a strong reaction to TB proteins. It makes TB infection less likely but does not rule it out, especially soon after exposure or with a weakened immune system. If you have had recent contact with someone with TB, your clinician may suggest testing again.
Common causes
- No TB infection
- Testing too soon after exposure, before the immune response has developed
- A weakened immune response from a condition or medicine
- A problem collecting or handling the blood sample
What can help
- Tell your clinician about any recent contact with someone who has TB
- Ask whether a repeat test is advised after recent exposure
If it’s high
A positive result means white blood cells in your sample reacted to TB proteins. This can be a sign of past or current infection with TB bacteria. The test can't tell latent infection from active TB disease, and a positive result does not by itself mean you have active TB or can spread it. Other tests are needed to confirm active disease.
Common causes
- Latent TB infection, meaning TB bacteria in the body without symptoms
- Active TB disease
- Rarely, infection with a few related bacteria, such as M. kansasii or M. marinum
- Rarely, a false-positive result
What can help
- Ask your clinician about a symptom check and a chest X-ray
- Ask whether you should be evaluated for treatment of latent TB infection
- Tell your clinician about any known TB exposure or earlier TB test results
What can affect your result
- Time since exposure to TB
- How the blood was collected and how fast it reached the lab
- The health of your immune system, including some medicines
- Age, pregnancy and some medical conditions: the manufacturer's original 2008 US labeling noted limited study in people under 17, in pregnancy and in hemophilia, and newer labeling may differ, so ask your clinician how it applies to you
- The blood must reach the lab quickly and at room temperature. Quest accepts a lithium or sodium heparin sample for up to 54 hours at room temperature and rejects refrigerated, frozen, centrifuged or hemolyzed samples.
- Incorrect collection or handling can damage the white blood cells and cause a false-negative result (manufacturer).
- A weak immune response, for example from some conditions or medicines, can make a result negative or invalid even when infection is present.
The immune response to TB bacteria can take a few weeks to develop after exposure, so a test too soon after contact can be negative.
When to talk to a clinician
A positive or borderline result needs follow-up with a clinician, but it is rarely an emergency. Contact a clinician promptly if you have a cough lasting three weeks or longer, coughing up blood, fever, night sweats or unexplained weight loss. For severe trouble breathing or chest pain, call 911 or your local emergency number.
Questions you could ask
- Why was this test chosen instead of a skin test or another TB blood test?
- Do I need a chest X-ray or other tests to check for active TB?
- Should I be evaluated for treatment of latent TB infection?
- Which symptoms should make me call you right away?
- Given my recent exposure or health conditions, do I need to be retested?
Questions people ask
Does the BCG vaccine cause a positive T-SPOT.TB?
It should not. The two TB proteins used in this test are absent from all BCG vaccine strains, according to the manufacturer's package insert.
What do the spot numbers on my report mean?
The lab counts spots for Panel A and Panel B and subtracts the spots in a control well. In the US labeling, 8 or more spots in either panel is positive, 4 or fewer in both is negative, and 5 to 7 is borderline. Go by the interpretation your lab prints, since your clinician reads it with your history.
What happens if my result is borderline or invalid?
The manufacturer recommends collecting another blood sample and testing again. If a result stays borderline, clinicians use other tests and your risk factors to work out your TB status.
Show the science and sources
For clinicians and researchers
T-SPOT.TB is an ELISPOT assay: peripheral blood mononuclear cells are separated and counted, then incubated with a phytohemagglutinin positive control, a nil control and two antigen panels (Panel A, ESAT-6, and Panel B, CFP-10). Secreted interferon-gamma is captured and counted as spots. In the US package insert approved by FDA (PMA P070006), the result is Positive if (Panel A minus Nil) and/or (Panel B minus Nil) is 8 spots or more, Negative if both are 4 spots or fewer (including values below zero), and Borderline (equivocal) when the highest value is 5, 6 or 7 spots, in which case retesting on another specimen is recommended. A nil control above 10 spots is invalid, and a positive control under 20 spots is invalid unless a panel is positive or borderline. Later studies cite the same criteria (8 or more positive, 5 to 7 borderline, 4 or fewer negative). Reports often list the Panel A, Panel B, nil and positive-control spot counts alongside the interpretation. The insert states that ESAT-6 and CFP-10 are absent from all BCG strains and from most non-tuberculous mycobacteria except M. kansasii, M. szulgai and M. marinum, that a positive result does not rule in active TB disease, and that a negative result does not exclude infection. Quest collects 6 mL of heparinized whole blood for adults and children 10 and older (9 mL for immunocompromised patients), with smaller volumes for younger children.
LOINC 74281-7 (primary), 71773-6, 74278-3, 74277-5. Specimen: whole-blood.
Sources
- T-SPOT.TB Package Insert (PI-TB-US), FDA PMA P070006, Oxford Immunotec (hosted by FDA)
- T-SPOT.TB (test code 37737), Quest Diagnostics
- Association between the extent of intrapulmonary spread on chest CT and false-negative results of T-SPOT.TB in pulmonary tuberculosis: a retrospective study, BMC Infectious Diseases (via PubMed Central) (2025)
- Signs and Symptoms of Tuberculosis, Centers for Disease Control and Prevention
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.