Testing for Tuberculosis: Skin Test (2024)

FAQs

What is a good TB skin test result? ›

An induration of less than 5 millimeters (mm) is a negative result. If you have symptoms or know you've been exposed to someone with TB, a doctor may recommend you get another test. If the induration is at least 5 mm, it is considered positive in people who: have had recent contact with a person with TB.

Your health care provider MUST check your arm 2 or 3 days after the TB skin test, even if your arm looks OK to you. If you have a reaction to the test, it will look like a raised bump. Your health care provider will measure the size of the reaction. If there is a bump, it will go away in a few weeks.

TB skin: Subjective results. Requires 2 separate appointments and manual results. Sensitivity of 79% and specificity of 97% with specificity being as low as 59% in BCG-vaccinated patients (high false positive).

You may have swelling or a bump where the testing fluid was injected. The health care worker will measure the bump or swelling and will tell you if your skin test reaction is positive or negative. If you are not able to return within 2 to 3 days, you will need to repeat the skin test again.

A measurement of 0 mm or a measurement below the defined cut point for each category is considered negative. * For employees who are otherwise at low risk for TB and who are tested as part of an infection control screening program at the start of employment, a reaction of ≥ 15 mm is considered positive.

Some persons may have a negative result from a TB skin test even though they are infected with TB bacteria. The reasons for these false-negative results may include, but are not limited to, the following: Anergy (inability to react to a TB skin test because of a weakened immune system)

After 2-3 days, you will be asked to see the nurse to have the area checked and the reaction measured. If there is no reaction after 2-3 days, you most probably have not been exposed to the TB germ. If there is a reaction, some redness and a small bump will appear on the arm.

Key Information About Your TB Skin Test

The testing solution/serum (Tubersol) is placed just under the top layer of your skin. A small bump (similar to a mosquito bite) appears when the serum is injected. This bump may disappear almost immediately or it may linger for a while.

The skin test reaction should be read between 48 and 72 hours after administration by a health care worker trained to read TST results. A patient who does not return within 72 hours will need to be rescheduled for another skin test. The reaction should be measured in millimeters of the induration (firm swelling).

How often are TB skin tests false positive? ›

In 18 studies involving 1,169,105 subjects, the absolute prevalence of false-positive TST from NTM cross-reactivity ranged from 0.1% to 2.3% in different regions.

The tuberculin skin test must be given and read by a professional who has been trained to do this, and has been informed of the most recent guidelines in tuberculosis control. Never allow anyone other than a nurse or physician to read a tuberculin skin test.

Results. Redness alone at the skin test site usually means you haven't been infected with TB bacteria. A firm red bump may mean you have been infected with TB bacteria at some time. The size of the firm bump (not the red area) is measured 2 to 3 days after the test to find out the result.

The face, hands, and legs are the commonest sites involved. The tuberculous chancre appears 1-4 weeks after inoculation, presenting initially as a firm red papule which becomes a painless shallow ulcer with a granular base and undermined edge. Sporotrichoid lesions and enlarged regional lymph nodes can develop.

It does not mean you have an active TB infection. The test may be seen as positive if the skin where you were injected is hard, raised, red, and swollen. But redness alone is not considered a positive test result. In many cases, a healthy immune system will surround the TB bacteria soon after you are infected.

In a healthy person whose immune system is normal, induration greater than or equal to 15 mm is considered a positive skin test. If blisters are present (vesiculation), the test is also considered positive. In some groups of people, the test is considered positive if induration less than 15 mm is present.

The tuberculosis (TB) blood test, also called an Interferon Gamma Release Assay or IGRA, is a way to find out if you have TB germs in your body. The TB blood test can be done instead of a TB skin test (Mantoux).

In people with no known risks of TB, 15 mm or more of firm swelling at the site indicates a positive reaction. People who were born outside the United States who have had a vaccine called BCG may have a false-positive test result.

Induration of 15 mm or more is considered positive in the following cases: Healthy individuals older than four years of age with a low likelihood of TB infection[14]