A TB test is a quick screening that checks whether your immune system has ever encountered tuberculosis bacteria. There are two main types: a skin test done at a clinic and a blood test sent to a lab. The skin test is more common and involves a tiny injection in your forearm, followed by a return visit 48 to 72 hours later so a healthcare worker can check your reaction. The blood test requires a single draw and no return visit.
The Skin Test (Mantoux Test)
The skin test uses a small amount of purified protein from TB bacteria, injected just under the surface of the skin on the inside of your forearm, about 2 to 4 inches below the elbow joint. The needle goes in at a very shallow angle, between 5 and 15 degrees, bevel facing up. The total amount of liquid is just 0.1 ml, so the injection is quick and creates a small, pale bump (called a wheal) that usually disappears within minutes to hours.
The test itself doesn’t contain live bacteria and can’t give you TB. It simply introduces proteins that your immune system will recognize if you’ve been exposed to tuberculosis before.
After the injection, you need to leave the site alone. Don’t cover it with a bandage, apply lotion, or scratch it. You can wash the area gently with water, but avoid wiping or scrubbing. If it itches, placing an ice cube or cold cloth on it is fine.
Coming Back to Read the Result
You must return to the clinic within 48 to 72 hours to have the site checked. This is non-negotiable: if you miss the window, the test has to be repeated. A trained healthcare worker will feel the injection site for any raised, hardened area (called induration) and measure it in millimeters. They’re measuring firmness, not redness. A red area without any firmness underneath doesn’t count as a reaction.
What counts as “positive” depends on your personal risk factors, not a single universal cutoff:
- 5 mm or more is positive for people with HIV, organ transplant recipients, those on immunosuppressive medications, recent close contacts of someone with active TB, or anyone with chest X-ray findings suggesting previous TB.
- 10 mm or more is positive for people born in countries where TB is common (parts of Asia, Africa, and Latin America), people who live or work in nursing homes, homeless shelters, or correctional facilities, children under 5, people with diabetes or severe kidney disease, and those with low body weight.
- 15 mm or more is positive for anyone with no known risk factors.
This tiered system exists because the consequences of missing a true infection are more serious for people at higher risk, so the threshold is set lower.
The Blood Test (IGRA)
The blood test works differently. A healthcare worker draws a standard blood sample from your arm, and the lab mixes it with synthetic proteins that mimic TB bacteria. If your white blood cells have encountered TB before, they react by releasing a signaling molecule called interferon-gamma. The lab measures either the concentration of that molecule or the number of cells producing it, depending on which version of the test is used.
The main advantage of a blood test is that it requires only one visit. There’s no injection site to protect and no return appointment to read results. Results typically come back within a few days, though the exact turnaround depends on the lab. Not every lab offers the test, so your provider may need to confirm availability before ordering it.
Which Test Is Better If You’ve Had the BCG Vaccine
Many people born outside the United States received the BCG vaccine against tuberculosis as children. This vaccine can trigger a false positive on the skin test, and there’s no reliable way to tell whether a skin test reaction came from the vaccine or from actual TB exposure. The reaction from the vaccine tends to fade over time, but periodic skin testing can actually boost it back up.
Blood tests don’t have this problem. The BCG vaccine does not cause false positives on blood tests, which is why blood tests are the preferred option for anyone who has been vaccinated with BCG, including children.
What Happens After a Positive Result
A positive result on either test means your immune system has encountered TB bacteria at some point. It does not mean you have active TB disease or that you’re contagious. Most people with a positive screening have what’s called latent TB infection, where the bacteria are present but dormant and causing no symptoms.
To figure out what’s actually going on, your provider will order a chest X-ray to look for signs of active disease in the lungs. You’ll also go through a medical history and physical exam. If the X-ray raises concerns, the next step is collecting sputum samples (mucus coughed up from the lungs) for laboratory testing. Culture testing of sputum is the gold standard for confirming active TB disease, though it can take several weeks for results because the bacteria grow slowly.
If your chest X-ray is clear and you have no symptoms, you likely have latent TB. Your provider will discuss whether treatment for latent infection makes sense based on your risk of the infection becoming active later in life.
Two-Step Testing for Healthcare Workers
If you’re starting a job in healthcare, a hospital, or another setting where TB testing is routine, you may be asked to do a two-step skin test. This involves getting a first skin test, reading it at 48 to 72 hours, and then repeating the entire process one to three weeks later.
The reason is something called the booster effect. If you were exposed to TB years ago or received the BCG vaccine as a child, your immune response may have faded enough that the first test comes back negative. But that first injection can “wake up” the immune memory, causing the second test to show a stronger, more accurate reaction. Without the two-step process, a future positive test might be mistaken for a new infection when it was really just a boosted old one. This baseline matters for workplaces that test employees regularly.