How to Read a Tuberculosis (TB) Skin Test

You read a TB skin test by feeling for firm swelling at the injection site and measuring it across the forearm. The width in millimeters, not the redness, is the result, and a trained health worker records it. The reading follows three steps¹.

1. Find the injection site on the inner forearm.
2. Feel for a firm, raised area.
3. Measure its width across the forearm in millimeters and record the number.

The reader checks the spot on the inner forearm where 0.1 mL of tuberculin was injected¹. One method uses the tip of a ballpoint pen, pushed gently toward the injection site at a 45 degree angle⁶. The pen stops at the edge of the induration⁶. A straight-edge millimeter ruler is used for the measurement⁶. Any dead skin, called necrosis, or fluid swelling, called edema, is noted but not used to decide the result⁵. Reliable reading takes standard procedures, training, supervision and practice¹. Under 2025 CDC guidance, only a designated health care provider with training should place and read it⁸. State and local public health authorities decide who may do so⁸.

A positive bump and a negative one are compared on a page of their own. The guide to the skin test bump: positive vs negative sets the two side by side.

A TB skin test should be read between 48 and 72 hours after it is placed. A test not read within 72 hours has to be placed again on another visit. This is why the skin test takes two visits, two or three days apart³. Placing a new test after a missed reading carries no risk of harm, although repeated tests can cause a boosted reaction¹. There is no reason not to repeat the test unless an earlier skin test caused a severe reaction¹. When someone has symptoms of TB disease, other tests do not wait for the skin test reading¹. The reaction starts 5 to 6 hours after the injection and peaks at 48 to 72 hours⁵. A TB blood test needs no return visit, so it suits people unlikely to come back for the reading⁹. The PPD skin test is named after the purified protein derivative injected at the first visit.

Induration is the firm, raised swelling that forms where TB tuberculin was injected. It is felt with the fingers and measured, while flat redness around it is ignored. A red patch with no firm swelling is read as 0 mm¹. The swelling comes from immune cells moving into the skin at the site⁵. In highly sensitive people the site can blister, ulcerate or form dead tissue⁵. A strongly positive reaction can leave a scar⁵. A skin test that is red but no bump is therefore a negative reading.

A positive TB skin test is an induration at or above the cutoff for your group: 5, 10 or 15 mm. The cutoff is lowest for people most likely to get sick and highest for people with no risk factors.

The result depends on both the size and the person's risk of infection or of progressing to disease¹. The 5 mm group also includes organ transplant recipients and people whose chest X-ray suggests old TB¹. It includes people taking prednisone at 15 mg a day or more for a month or longer¹. The 10 mm group adds people who misuse drugs or alcohol and mycobacteriology lab workers¹. Residents and staff of high-risk group settings belong to it too¹. It also covers people with diabetes or severe kidney disease, and people under 90% of ideal body weight¹. For a low-risk person tested at the start of a job, 15 mm or more counts as positive¹. A reading can be falsely positive after BCG or after infection with other mycobacteria¹. A wrong measurement or the wrong antigen can cause one too¹. A positive tuberculosis test does not show whether the infection is latent or has become active disease². Latent infection is not contagious, CDC said in 2025. A chest X-ray and sputum tests check for TB disease².

0 mm induration means no firm swelling formed, so the skin test is negative. A negative test makes TB infection unlikely but does not rule out TB disease. A skin test can be falsely negative within 8 to 10 weeks of infection and in babies under 6 months¹. It takes 2 to 8 weeks after infection for the skin test to show a reaction¹. A weakened immune system or a recent live-virus vaccine can cause a false negative¹. So can a recent viral illness such as measles, or a test given or measured wrongly¹. This is especially true in people who are severely ill or have HIV¹. With symptoms of active TB, a negative tuberculosis (TB) test is followed by more tests².

A two-step TB skin test is read like any skin test, twice, each time 48 to 72 hours after placement. The second test is placed 1 to 3 weeks after a negative first reading. A positive second test shows a boosted reaction to an older infection, not a new one³. That person is still a candidate for treatment of latent TB infection³. A negative second test means the baseline result is negative³. Boosting happens in older adults whose response has faded over the years, and in people who had BCG or other mycobacteria¹. The US has two tuberculin products, Tubersol and Aplisol⁷. A CDC report from 2019 says switching between them in repeat testing might look like a new conversion⁷. The two-step tuberculosis (TB) skin test sets a baseline for people tested again later.

These questions cover who reads the test, timing, BCG and the blood test.