Positive Tuberculosis (TB) Test

A positive TB skin test looks like a firm, raised bump at the injection site on the forearm. It counts as positive only at the swelling size set for your risk group; redness is ignored. The swelling is measured across the arm in millimeters 48 to 72 hours after the test¹. A blood test shows no mark at all, because it is read in a lab⁴. A ring of redness often surrounds the firm center (Youssef, 2005). The reaction usually starts within 24 hours and peaks between 48 and 72 hours. The reader finds the edges by sweeping the fingertips over the arm in all four directions and marks the widest points. Slight bruising, a rash or redness at the site is common and does not by itself mean a positive result.

After a positive result, a chest X-ray is the usual next test. The guide to the chest X-ray for tuberculosis explains what it shows.

A positive TB test means your immune system has reacted to TB germs, so the germs are in your body. It cannot show on its own whether you have inactive TB or active TB disease. Further tests decide which, and both are treatable². When a medical evaluation finds no sign of TB disease, the diagnosis is inactive TB³. A CDC page from 2025 estimates that up to 13 million people in the US live with inactive TB. The page on tuberculosis test results covers each kind of result.

A positive TB blood test means your blood reacted to TB proteins in the lab, showing TB germs in your body. The two US blood tests, QuantiFERON-TB Gold Plus and T-SPOT.TB, both measure interferon-gamma released by white blood cells. The lab mixes the blood with ESAT-6 and CFP-10⁴. These are proteins found in TB germs but absent from most other mycobacteria⁴. The blood test takes one visit, against two for the skin test⁷. QuantiFERON reads positive when a TB tube minus the nil tube is 0.35 IU/mL or more. A higher value does not mean a later stage of infection or a greater chance of disease. An indeterminate, borderline or invalid result gives no useful answer⁴. Repeating the blood test or doing a skin test may help⁴. The page on a positive tuberculosis blood test goes through the report line by line.

A positive TB skin test is a firm swelling of at least 5, 10 or 15 mm, depending on who you are. It is 5 mm for the highest-risk groups, 10 mm for other risk groups and 15 mm for everyone else.

The size of the swelling does not show how likely current TB disease is. The same swelling, for example, can be positive for a person with HIV but negative for someone with no known risk factor. The steps to read a tuberculosis skin test in millimeters explain the measuring.

A TB skin test result counts only when a trained health care provider reads it, not when you check it yourself. If nobody reads it within 72 hours, you need a new skin test, placed as soon as possible. The record lists the exact millimeters of firm swelling, written as 0 mm when there is none¹. It also states whether that size is positive for your risk group¹. Who may read the test is set by each state, so state and local health departments say who is authorized.

A TB test is usually positive because you breathed in TB germs from someone with active TB disease, sometimes years earlier. Your immune system needs 2 to 8 weeks after infection before a skin or blood test can pick it up. The germs leave a sick person through coughing, speaking or singing⁵. They can stay in the air for several hours⁵. TB is not caught by shaking hands, sharing food or drink, or touching bed linens or toilet seats⁵. After a known exposure, a negative test is repeated 8 to 10 weeks after the last contact¹. The page on how tuberculosis spreads covers who can pass it on.

Yes, a TB test can be falsely positive, giving a positive result in a person who does not have TB germs. The BCG vaccine is the best-known cause, and a skin test cannot reliably tell a BCG reaction from infection. The skin test is read as if the person were unvaccinated¹. The blood test is not affected by BCG, so it is preferred for people who had the vaccine⁴. CDC guidance from 2025 also lists infection with other mycobacteria, a wrong measurement or reading, and the wrong antigen as causes¹. Three related germs, M. kansasii, M. szulgai and M. marinum, carry the same proteins and can make a blood test falsely positive. A low-risk person with a positive test gets a second test and counts as infected only if both are positive⁹. US health care workers were retested this way⁹. The second test was negative in 48% of blood tests and 62% of skin tests⁹. A past BCG vaccine is the first thing a reader asks about.

No, a positive TB test causes no illness, and people with inactive TB have no symptoms and do not feel sick. Symptoms instead point to active TB disease, such as a bad cough that lasts 3 weeks or longer. These are the other symptoms of active TB⁸.

• Chest pain
• Coughing up blood or sputum
• Weakness or tiredness
• Weight loss and no appetite
• Chills and fever
• Night sweats

On the arm, mild itching, swelling or irritation is common and is no reason to avoid future tests. In 1% to 2% of positive results the site blisters, which usually settles on its own (Youssef, 2005). The list of early signs of tuberculosis covers symptoms outside the lungs too.

No: a positive TB test does not make you contagious. People with inactive TB cannot spread TB germs, because active TB disease, usually of the lungs or throat, is what spreads. A person with a positive test, no symptoms and a normal chest X-ray usually has inactive TB³. Even a person newly exposed to TB germs cannot pass them on right away. People who spent time with someone who has inactive TB do not need a TB test³. Treating latent tuberculosis (TB) infection keeps it that way.

After a positive TB test, more tests check whether the germs are inactive or causing disease. The first is a chest X-ray, followed by a sputum test if the X-ray or symptoms point to active TB.

1. A chest X-ray, which generally rules out lung TB when it is normal and there are no symptoms.
2. Sputum smear and culture when needed, from at least three samples collected 8 to 24 hours apart.
3. Treatment: for inactive TB, 3HP weekly for 3 months (12 doses), 4R daily for 4 months or 3HR daily for 3 months.

An abnormal X-ray cannot by itself confirm TB disease, because other illnesses can look like it. Treatment for inactive TB is 90% effective at preventing TB disease⁶. Even without a known risk factor, treatment may be offered once a blood test is positive⁶. The same goes for a skin test at 15 mm or larger⁶. The guide to TB diagnosis step by step follows each test.

Positive and negative TB skin tests differ in the size of the firm swelling, not in how red the arm looks. A negative test shows no firm swelling, or swelling smaller than the cutoff for your risk group. A flat red patch with no firm bump is read as negative¹. A negative result may still lead to more tests if you have symptoms, have HIV or were recently exposed to TB⁷. The positive vs negative TB skin test bump page shows the two side by side.

These short answers cover work, lasting results, risk and two special cases.