A TB test result tells you whether you’ve been infected with tuberculosis bacteria. There are two main types of TB tests, a skin test and a blood test, and each reports results differently. A positive result on either test usually means you’ve been infected with TB bacteria at some point, but it does not necessarily mean you have active TB disease. Further testing is always needed after a positive result to determine what’s actually going on.
How the Skin Test Works
The TB skin test (sometimes called a PPD or Mantoux test) involves a small injection just under the skin of your forearm. A healthcare provider injects a tiny amount of protein derived from TB bacteria, then you return 48 to 72 hours later to have the injection site checked. What matters is not redness or discoloration at the site, but whether a firm, raised bump (called induration) has formed and how large it is in millimeters.
Your provider measures the bump across its widest point. The size that counts as “positive” depends on your personal risk factors. People with weakened immune systems, such as those with HIV or organ transplant recipients, have a lower threshold for a positive reading. People with no known risk factors need a larger bump before the result is considered positive. If you miss the 48-to-72-hour reading window, the test typically needs to be repeated because the results can no longer be accurately interpreted.
How the Blood Test Works
TB blood tests, known as IGRAs, require a single blood draw with no return visit. The two main versions are QFT-Plus and T-SPOT. Your blood sample is mixed with proteins that mimic TB bacteria, and the lab measures how your immune cells respond. If your immune system recognizes those proteins, it suggests you’ve been exposed to TB.
QFT-Plus results come back as positive, negative, or indeterminate. T-SPOT results are reported as positive, negative, borderline, or invalid. A positive result usually means TB infection. A negative result means infection is unlikely, though it can’t be completely ruled out, especially if you have symptoms or a weakened immune system. An indeterminate, borderline, or invalid result simply means the test didn’t produce usable information. In that case, repeating the blood test or doing a skin test instead is the typical next step.
What a Positive Result Actually Means
A positive TB test, whether skin or blood, does not mean you’re sick. It means your body has encountered TB bacteria at some point. The critical next question is whether you have latent TB infection or active TB disease, because the two are very different situations.
With latent TB infection, you carry a small number of TB bacteria that are alive but inactive. You feel completely fine, have no symptoms, and cannot spread TB to anyone. With active TB disease, the bacteria are multiplying and causing damage, typically in the lungs. You usually feel sick, may have symptoms like a persistent cough, fever, night sweats, or unexplained weight loss, and you may be contagious.
After a positive test result, you’ll need a chest X-ray to look for signs of active disease in the lungs. If the X-ray raises concerns, sputum samples (mucus coughed up from deep in the lungs) are collected and tested for live TB bacteria. This combination of imaging and lab work is what separates a latent infection from active disease.
What a Negative Result Means
A negative result is generally reassuring. It means TB infection is unlikely. However, no test is perfect. A negative result can occasionally occur even when someone is infected, particularly in people with suppressed immune systems who may not mount a strong enough response for the test to detect. Recent infections can also be missed because the immune system needs several weeks after exposure to develop a detectable response.
If you have symptoms consistent with TB or a known recent exposure to someone with active TB, a negative test result doesn’t automatically clear you. Your provider may recommend additional testing or monitoring.
BCG Vaccine and False Positives
The BCG vaccine, given at birth or in childhood in many countries outside the United States, can complicate skin test results. Because the vaccine contains a related strain of bacteria, your immune system may react to the skin test even if you’ve never had a true TB infection. This is one reason TB blood tests are often preferred for people who received the BCG vaccine. Blood tests are not affected by prior BCG vaccination, making them more reliable in this situation.
Latent TB: What Happens Next
If your chest X-ray is normal and you have no symptoms, you’re diagnosed with latent TB infection. This is not an emergency, but it does matter. About 5 to 10 percent of people with untreated latent TB will eventually develop active disease at some point in their lives, with the highest risk in the first two years after infection. Treatment for latent TB involves a course of medication that kills the dormant bacteria before they can activate. Treatment regimens vary in length, but shorter options lasting three to four months are now widely available.
If you have latent TB and choose not to treat it, the CDC recommends annual screening for symptoms of active disease. You would not need repeat skin or blood testing, since those will likely remain positive indefinitely. Instead, the focus shifts to watching for new symptoms like a prolonged cough, fever, or unintentional weight loss.
Who Needs TB Testing
TB testing isn’t recommended as a routine screening for everyone. It’s targeted toward people with a meaningful chance of infection or those whose infection would carry higher consequences. This includes people who have spent time in countries where TB is common, those who have been in close contact with someone diagnosed with active TB, healthcare workers, and people with conditions that weaken the immune system.
For healthcare workers specifically, the CDC recommends TB screening at the time of hire, including a risk assessment, symptom evaluation, and either a skin or blood test. Routine annual testing is no longer recommended unless there’s a known exposure or evidence of ongoing transmission at a facility. All healthcare workers should receive annual TB education regardless of their test history.
Skin Test vs. Blood Test: Which Is Better
Both tests detect immune responses to TB bacteria rather than the bacteria themselves, and both are considered acceptable options. The skin test is cheaper and widely available but requires two visits and can be affected by BCG vaccination. Blood tests need only one visit and aren’t influenced by prior vaccination, but they cost more and require a lab with the right equipment.
Blood tests tend to be preferred for people who received the BCG vaccine, those unlikely to return for a skin test reading, and people who need serial testing over time. Skin tests remain a practical choice in many clinical settings, particularly for large-scale screening. Your provider will typically choose the test that makes the most sense given your medical history and circumstances.