How does a TB tine test work?

The tuberculin skin test (TST), also known as the Mantoux test, is a method of testing for tuberculosis (TB) infection. Here is a simplified explanation of how it works:

1. Preparation: A healthcare professional cleans a small area of the skin, usually on the inner side of the forearm.

2. Injection: Using a small needle, a tiny amount of tuberculin (a purified protein derivative from the tuberculosis bacteria) is injected just under the top layer of the skin. This injection should not be painful.

3. Reaction Development: After the injection, the area where the tuberculin was injected is monitored closely. Over the next 48 to 72 hours, a reaction will develop if the person has been exposed to TB bacteria or has TB infection.

4. Reading the Test: The tuberculin skin test is read by a healthcare professional, usually 48 to 72 hours after the injection. They will examine the injection site and measure the size of any reaction.

- Negative Reaction: If there is no swelling or redness (induration) at the injection site, or if the swelling is less than 5 millimeters (mm), the test is considered negative. This indicates that the person either has not been exposed to TB bacteria or does not have an active TB infection.

- Positive Reaction: If there is a raised, firm swelling at the injection site that is 5 mm or more in diameter, the test is considered positive. This does not necessarily mean the person has active TB disease. It may indicate a TB infection, in which case further evaluation is needed to confirm an active infection.

5. Interpretation: The interpretation of the tuberculin skin test results takes into account the person's risk factors, medical history, and any symptoms they may have. A positive test may lead to additional testing, such as a chest X-ray, sputum test, or blood test, to determine the presence of active TB disease and decide on the appropriate treatment.

6. False-positive and False-negative Reactions: The tuberculin skin test is not foolproof. There can be false-positive reactions in people who have been vaccinated with the BCG vaccine or have been exposed to other types of mycobacteria. False-negative reactions can occur in people with weakened immune systems or in certain stages of TB infection.

It's important to note that tuberculin skin tests should be performed and interpreted by trained healthcare professionals.