Tuberculosis (TB) Test Results

TB skin test results take 2 to 3 days, because the reaction is read 48 to 72 hours after placement. Blood test results can be ready within 24 hours but may take several days, depending on the lab. A skin test left unread past 72 hours has to be placed again¹. The lab processes QuantiFERON-TB Gold Plus blood within 16 hours of the draw, and T-SPOT.TB blood within 8 to 32 hours². Two-step testing takes longer, because the second skin test goes in 1 to 3 weeks after the first³.

If your result is positive, the next steps start straight away. The page on a positive TB test walks through them in order.

TB skin test results are read by measuring, in millimeters, the firm swelling at the injection site. Redness is not measured, and whether a size counts as positive depends on your risk group. A trained health care worker does the reading, not the person who had the test¹. A past BCG vaccine leaves the cutoffs unchanged, and the reaction is judged as if the vaccine was never given¹.

The measuring itself is set out step by step in how to read a tuberculosis skin test.

A skin test result is a firm bump, or none, on the forearm, and a blood test result is a lab report. The QuantiFERON report lists four values, Nil, TB1, TB2 and Mitogen, in IU/mL, beside the final result. When the skin test is placed correctly, it first raises a pale wheal 6 to 10 mm wide¹. At the reading, the firm swelling is measured across the forearm, not along it¹. The lab subtracts the Nil value from the TB1, TB2 and Mitogen values to remove background⁸. QuantiFERON is positive when TB1 or TB2 minus Nil reaches 0.35 IU/mL or more and at least 25% of Nil⁸. That rule applies only while Nil stays at 8.0 IU/mL or less⁸. A T-SPOT.TB report counts spots instead, each spot one blood cell making interferon-gamma².

A negative TB test result means the test found no sign of TB infection. It makes TB unlikely, but more tests may follow if you have symptoms, HIV or a recent exposure. Infection in the past 8 to 10 weeks can leave a skin test negative, as can an age under 6 months¹. It can miss TB in people who are very ill¹. A recent live-virus vaccine, such as measles or varicella, or a recent viral illness can give a false negative too¹. So can a test that was placed or measured the wrong way¹. A negative blood test has the same meaning, with the same reasons for further testing⁵. More detail sits on the page about a negative tuberculosis test.

TB blood test results report whether the lab found an immune reaction to TB germs in your blood. QuantiFERON-TB Gold Plus reports positive, negative or indeterminate, and T-SPOT.TB adds borderline and invalid. An indeterminate or invalid result answers neither way, so it must never be taken as negative². The BCG vaccine leaves a blood test result unchanged². A higher QuantiFERON number does not mean a later stage of infection or a bigger chance of falling ill⁸. After a result with no answer, the blood test can be repeated, or a skin test done instead². Rare TB relatives, such as M. kansasii, M. szulgai and M. marinum, might make a blood test falsely positive (MMWR, 2010). The guide to QuantiFERON results explained goes through each outcome.

A record of a TB test result is a written copy showing the test, the date and the result. CDC guidance from 2025 advises keeping a written record of a positive result, because later tests can stay positive after treatment. For a skin test, the record gives the reading in millimeters, not just positive or negative³. After a documented skin test in the past 12 months, one new test counts as step two of two-step testing (CDC, 2023). After treatment for latent TB, a record of the finished treatment is kept to show at later tests (CDC, 2025). The page on the tuberculosis test form shows what such a record holds.

What you should do after your TB test results depends on whether the result is positive or negative. A positive result needs a chest X-ray and often a sputum test, and a negative result usually needs nothing more. People tested regularly at work, such as health care workers, have repeat tests over time³. Under CDC guidance from 2019, health care workers with no known exposure are not retested every year after their baseline test. After a positive result, a provider decides between inactive TB, which is not contagious, and active TB disease, then discusses treatment⁵. Treatment for latent TB infection is 90% effective at preventing TB disease¹. Anyone facing testing again can compare the options in the tuberculosis test guide.

A chest X-ray after a positive TB test looks for lung changes that point to active TB disease. A normal X-ray with no symptoms generally rules out lung TB, and points to latent TB infection. An abnormal X-ray cannot confirm TB disease on its own, because other illnesses can look the same⁷. In adults without HIV, lung TB usually makes the X-ray abnormal, showing infiltrates or cavities⁷. In people with HIV and in children, lung TB can leave the X-ray normal or only subtly changed⁷. Rare exceptions to the normal-X-ray rule occur in people with HIV whose immune system has not recovered on treatment⁶. An X-ray suggesting old, healed TB puts a person in the 5 mm skin test group¹. The typical patterns are described in what TB looks like on a chest X-ray.

TB sputum test results show whether TB germs are found in mucus coughed up from deep in the lungs. A positive culture confirms active TB disease, while a negative smear or culture does not fully rule it out. At least three samples are collected 8 to 24 hours apart, one of them early in the morning⁶. Smear results are expected within 24 hours of collection⁶. A NAAT, a DNA test, finds TB within hours, while culture takes a week or more⁶. Culture is the gold standard, and every sample is cultured whatever the smear shows⁷. Culture results take 2 to 6 weeks (CDC, 2024). A positive culture then goes on to testing for drug resistance⁶. Some people with negative cultures are still diagnosed with TB disease from their symptoms and response to treatment⁷. How to give a good sample is covered on the page about the sputum test for tuberculosis.

These short answers cover how long results last, false negatives, cutoffs, copies and same-day results.