A TB blood test is a simple blood draw that checks whether your immune system has been exposed to the bacteria that cause tuberculosis. Officially called an interferon-gamma release assay (IGRA), it works by measuring how your white blood cells react when mixed with proteins from TB bacteria in a lab. Results typically come back within 24 to 48 hours, and unlike the traditional skin test, you only need one visit to complete it.
How the Test Works
After a standard blood draw, the lab mixes your blood sample with synthetic proteins that mimic antigens found on TB bacteria. If you’ve been infected with TB at some point, your immune system will have developed specific white blood cells that recognize those proteins. Those cells respond by releasing a chemical signal called interferon-gamma. The lab measures how much of that signal your blood produces and compares it against control samples to determine whether you’ve been exposed.
This immune response typically takes six to eight weeks to develop after an initial TB infection. That means if you were exposed to TB very recently, a blood test taken too soon could come back negative even though you’re actually infected. In those cases, your provider may recommend retesting after a waiting period.
Two FDA-Approved Versions
Two TB blood tests are available in the United States, and both work on the same basic principle. The QuantiFERON-TB Gold Plus (made by Qiagen) uses two tubes of TB antigens designed to stimulate different types of immune cells. The T-SPOT.TB (made by Oxford Immunotec) takes a slightly different approach: it isolates your white blood cells and counts the individual cells that react to TB proteins, reporting results as a number of “spots” on a testing plate. Your provider chooses which test to order based on lab availability and cost, and both are considered reliable for screening purposes.
What Your Results Mean
A TB blood test returns one of three results: positive, negative, or indeterminate.
- Positive means your immune system recognizes TB bacteria, which confirms you’ve been infected at some point. It does not tell you whether you have latent (inactive) TB or active TB disease. Additional testing is needed to make that distinction, usually a chest X-ray and possibly a sputum test where you cough up a sample for the lab to examine.
- Negative means the test found no significant immune response to TB proteins. In most cases, this means you’re not infected. However, if you were exposed within the past six to eight weeks, a false negative is possible because your immune system may not have mounted a detectable response yet.
- Indeterminate means the test couldn’t produce a clear answer. This can happen when your immune system is weakened by conditions like HIV, certain medications (such as those that suppress the immune system after an organ transplant), or very young or very old age. Technical issues with blood handling or processing can also cause indeterminate results. If this happens, your provider will typically repeat the test or switch to a skin test.
An important point many people miss: a positive result almost always leads to the same next step regardless of which test was used. You’ll get a chest X-ray to look for signs of active disease in the lungs. If the X-ray is clear, you likely have latent TB, meaning the bacteria are in your body but dormant and not contagious. If the X-ray shows abnormalities, further testing confirms whether you have active TB disease that needs immediate treatment.
Who Should Get a Blood Test Instead of a Skin Test
The CDC recommends blood tests as the preferred screening method for anyone 5 years of age or older who has received the BCG vaccine. BCG is a tuberculosis vaccine given routinely in many countries outside the United States, and it’s the main reason blood tests have become so widely used. The traditional TB skin test reacts to BCG vaccination, often producing a false positive result in vaccinated people. The blood test avoids this problem entirely because the proteins it uses are found in TB bacteria but are absent from the BCG vaccine strain.
Blood tests are also the more practical option for people who can’t easily return for a second appointment. The skin test requires you to come back 48 to 72 hours after the injection so a provider can read the reaction on your arm. If you miss that window, the test is wasted and has to be repeated. The blood test eliminates that issue since everything happens in the lab after a single visit.
For children under 5, the CDC still recommends the skin test as the primary screening method, though some providers use blood tests in younger children on a case-by-case basis. Skin tests remain an acceptable alternative for anyone when blood tests are unavailable, too costly, or logistically difficult to arrange.
How Accurate Is It
No TB screening test, whether blood or skin, is perfect. A large meta-analysis published in The Lancet’s eClinicalMedicine found that in lower-incidence countries (like the United States), the QuantiFERON blood test had a sensitivity of about 65% and a specificity of about 74% for predicting who would go on to develop active TB. The skin test performed similarly, with 63% sensitivity and 76% specificity. These numbers mean both tests catch roughly two-thirds of people who will eventually develop active disease, and both correctly rule out about three-quarters of people who won’t.
These figures reflect the tests’ ability to predict future active disease, which is a high bar. For simply detecting whether someone has been exposed to TB bacteria, both tests perform better. The key takeaway is that neither test alone is a definitive diagnosis. A positive result on either test is a starting point that triggers additional evaluation, not a final answer.
What the Experience Is Like
Getting a TB blood test feels like any other blood draw. A technician draws one or more tubes of blood from your arm, and you’re done in a few minutes. There’s no injection under the skin, no waiting period, and no return visit to have a result “read.” Results are typically available within one to three days depending on your lab.
You don’t need to fast or prepare in any special way. If you’re taking medications that suppress your immune system, let your provider know beforehand since this can affect the reliability of the results. The same applies if you’ve been ill recently, as your immune response may not be at full strength.