TB skin tests

Tuberculin skin test

The Tuberculin skin test, also known as the Mantoux test or PPD test, is a diagnostic tool used to detect tuberculosis (TB) infection. It is a simple and cost-effective test that helps identify individuals who have been exposed to the bacteria that cause TB.

Procedure

The Tuberculin skin test involves injecting a small amount of purified protein derivative (PPD), a substance derived from the bacteria that cause TB, into the top layer of the skin. This is usually done on the forearm. After the injection, the site is observed for any reaction.

Interpretation

The test is read between 48 and 72 hours after the injection. A positive reaction is indicated by the presence of a raised, red bump at the injection site. The size of the bump is measured and recorded in millimeters. The larger the bump, the more likely it is that the individual has been infected with TB.

False Positive and False Negative Results

It is important to note that the Tuberculin skin test can produce both false positive and false negative results. A false positive occurs when the test indicates TB infection, but the individual is not actually infected. This can happen if the person has received the Bacillus Calmette-Guérin (BCG) vaccine, which is used to prevent TB in some countries. The BCG vaccine can cause a positive reaction on the Tuberculin skin test.

On the other hand, a false negative occurs when the test indicates no TB infection, but the individual is actually infected. This can happen if the person has a weakened immune system, such as those with HIV/AIDS, or if the test is performed too soon after exposure to the bacteria.

Importance

The Tuberculin skin test is an important tool in the diagnosis of TB infection. It helps identify individuals who have been exposed to the bacteria and may require further testing or treatment. Early detection and treatment of TB is crucial in preventing the spread of the disease and reducing its impact on public health.

Follow-up Testing

If the Tuberculin skin test is positive, further testing is usually recommended to confirm the diagnosis. This may include a chest X-ray, sputum culture, or molecular tests such as the polymerase chain reaction (PCR). These tests help determine if the individual has active TB disease or latent TB infection.

Limitations

While the Tuberculin skin test is a valuable diagnostic tool, it does have some limitations. It cannot differentiate between active TB disease and latent TB infection. Additional tests are required to make this distinction. Furthermore, the test may not be as accurate in individuals with compromised immune systems.