A TB test checks whether your immune system has ever encountered tuberculosis bacteria. It does this by looking for a specific immune response, either through a small injection under the skin or a blood draw. Neither test looks for the bacteria directly. Instead, both detect whether your body’s immune cells recognize TB proteins, which only happens if you’ve been exposed to the infection at some point.
The Two Types of TB Tests
There are two main ways to test for TB: a skin test (sometimes called a Mantoux test or PPD test) and a blood test (called an interferon-gamma release assay, or IGRA). Both measure immune memory rather than the bacteria itself, but they do it in very different ways and on very different timelines.
The skin test has been around for decades and is the version most people picture when they think of TB screening. The blood test is newer and preferred in certain situations, particularly for people who received the BCG vaccine (a TB vaccine given routinely in many countries outside the United States). The CDC recommends blood tests as the preferred method for anyone 5 years and older who has had BCG, since the vaccine can interfere with skin test results. For children under 5, the skin test remains the standard recommendation.
How the Skin Test Works
A healthcare provider injects a tiny amount of purified protein from TB bacteria, called PPD, just beneath the surface of your skin on the inside of your forearm. The dose is small: 0.1 milliliters, delivered with a fine needle angled at 5 to 15 degrees so it sits in the top layer of skin rather than going deeper into muscle. If done correctly, a small, pale bump (called a wheal) forms immediately at the injection site. That bump is normal and disappears within minutes.
What happens next is the actual test. If your immune system has encountered TB bacteria before, it produced specialized white blood cells called T cells that remember those proteins. When those T cells detect the injected proteins in your skin, they trigger an inflammatory response at the site. This is a delayed-type hypersensitivity reaction, meaning it doesn’t happen right away. It builds over 48 to 72 hours, which is why you have to return to your provider two to three days later to have the site read.
At the reading appointment, your provider feels the injection site for a firm, raised area called induration. This is different from simple redness. It’s the thickened tissue created by immune cells flooding to the area. They measure the diameter of that firm bump in millimeters. Whether that measurement counts as “positive” depends on your individual risk factors. People with weakened immune systems or known TB exposure have a lower threshold for a positive result than someone with no risk factors.
How the Blood Test Works
The blood test skips the waiting period entirely. A provider draws your blood, and the lab mixes it with synthetic proteins that mimic specific components of TB bacteria. The two proteins used, called ESAT-6 and CFP-10, were chosen because they’re found in the TB bacterium but are absent from the BCG vaccine strain and from most other related bacteria. This is the key advantage over the skin test: the blood test is far less likely to react to a prior BCG vaccination.
If you’ve been infected with TB, certain white blood cells in your blood sample will recognize those proteins and release a signaling molecule called interferon-gamma. The lab measures how much of this molecule the cells produce. A strong response means your immune cells have seen TB before. A weak or absent response means they likely haven’t. The two commercially available versions of this test, QuantiFERON-TB Gold Plus and T-SPOT, both use this same basic principle but measure the interferon-gamma response slightly differently.
After infection, the immune system typically needs six to eight weeks to develop this response. So both the skin test and the blood test can miss very recent infections if the exposure happened less than two months ago.
What a Positive Result Actually Tells You
This is where many people get confused. A positive result on either test means your body has been exposed to TB bacteria and your immune system recognizes it. It does not tell you whether you have an active, contagious infection or a latent one where the bacteria are present but dormant and causing no symptoms.
Most people who test positive have latent TB. The bacteria are in their body but walled off by the immune system. They feel fine, have no symptoms, and can’t spread TB to anyone. A smaller number have active TB disease, which causes symptoms like a persistent cough, fever, night sweats, and weight loss.
A positive screening test is always just the first step. Your provider will follow up with a chest X-ray and possibly sputum samples (mucus coughed up from the lungs) to look for signs of active disease. If no evidence of active TB turns up, you’ll be diagnosed with latent TB. If the evaluation does find active disease, treatment is more intensive and longer.
How Accurate Are These Tests?
Neither test is perfect. A large meta-analysis published in The Lancet found that in countries with lower TB rates, both the skin test and the QuantiFERON blood test had a sensitivity around 63 to 65 percent for predicting who would go on to develop active TB. Specificity hovered around 74 to 76 percent. In practical terms, this means both tests miss some infections and occasionally flag people who don’t actually have TB.
False positives on the skin test are more common in people who received the BCG vaccine, since the vaccine uses a related (but not identical) strain of bacteria. The immune system can react to the skin test proteins even without true TB infection. Blood tests largely avoid this problem because of the specific antigens they use, which is why the CDC prefers them for BCG-vaccinated individuals.
False negatives can occur with either test in people whose immune systems are suppressed, whether from HIV, certain medications, or other conditions. A weakened immune system may not mount enough of a response to register as positive, even if TB bacteria are present.
What to Expect During Each Test
The skin test requires two visits. The first is the injection itself, which takes just a few minutes and feels like a brief pinch. You’ll need to keep the area clean and avoid scratching or covering it with a bandage. Then you return 48 to 72 hours later for the reading. If you miss that window, the test typically needs to be repeated because the reaction may have faded.
The blood test requires only one visit, since it’s a standard blood draw. Results usually come back from the lab within a few days, though turnaround times vary by facility. For people who have trouble scheduling a return visit or who might not come back for their reading, the blood test is often more practical.
Why You Might Need One Over the Other
Your provider will choose based on your age, vaccination history, and circumstances. Blood tests are preferred for anyone who received the BCG vaccine, people who need quick results without a follow-up appointment, and those who are being tested as part of workplace screening where return visits are difficult to coordinate. Skin tests remain the go-to for young children under 5, partly because there’s more data on how to interpret skin test results in that age group.
In some cases, both tests may be used together. If one result is unclear or doesn’t match your risk level, a second test using the other method can help clarify things. Neither test alone is definitive for diagnosing TB disease, so additional evaluation always follows a positive result regardless of which screening method was used.