The Mantoux test, also known as the tuberculin skin test (TST), is a diagnostic tool used to detect latent tuberculosis (TB) infection. It is a simple and cost-effective test that has been widely used for over a century to screen for TB infection, especially in high-risk populations. The test involves the intradermal injection of a small amount of purified protein derivative (PPD) into the skin and the subsequent observation of the immune response.
When a person is infected with Mycobacterium tuberculosis, the bacterium that causes TB, their immune system responds producing a specific type of response called delayed-type hypersensitivity. This response involves the activation of T cells, which release certain chemicals that cause an inflammation at the site of the PPD injection. This inflammation is measured and interpreted to determine whether a person has been exposed to TB.
To perform the Mantoux test, a healthcare professional cleans the inner side of the forearm with alcohol. They then use a small needle to inject 0.1 mL (5 tuberculin units) of PPD just beneath the surface of the skin. After the injection, the healthcare professional marks the site of injection with a pen, and the person being tested is advised to keep the area dry and not scratch it.
The test should be read 48 to 72 hours after the injection. The healthcare professional examines the area of injection and measures the size of the induration, which is the hard swelling or bump that has formed. The size of the induration, rather than any redness or itchiness, is what is important for interpretation.
The size of the induration is measured across the forearm in millimeters using a ruler. A positive reaction is defined as an induration of 5 millimeters or more in the following high-risk populations:
– People who have come into contact with someone known to have active TB disease.
– People with HIV infection or other conditions that weaken their immune system.
– People with fibrotic changes on a chest X-ray consistent with prior TB.
– Organ transplant recipients and others who are taking immunosuppressive medications.
A positive reaction is also considered significant in people who have other risk factors, including recent immigrants from areas where TB is common, intravenous drug users, healthcare workers, and residents or employees of high-risk congregate settings.
Interpreting the test results can be challenging as false-positive and false-negative results can occur. False-positive results can happen if a person has been previously vaccinated with Bacillus Calmette-Guérin (BCG), a vaccine for TB, as it can cause a cross-reactive response. Certain environmental mycobacteria can also cross-react with the PPD, leading to false positives.
False-negative reactions can occur in people with weakened immune systems, such as those with HIV infection, or in people who have been infected with M. tuberculosis within the past 8-10 weeks.
If the test is positive, further evaluation is required to determine whether the person has latent TB infection or active TB disease. This may involve additional tests such as a chest X-ray, sputum culture, or molecular testing for TB. Contact tracing and screening of close contacts may also be necessary.
It is important to note that the Mantoux test is not fully reliable as a standalone test for diagnosing TB infection. It is a screening tool that detects the immune response to M. tuberculosis. Confirmation of infection and determination of disease status require further clinical evaluation and laboratory tests.
The Mantoux test, or tuberculin skin test, is a valuable diagnostic tool used to detect latent tuberculosis infection. It involves the intradermal injection of PPD and observation of the immune response. Interpretation of the test results is based on the size of the induration observed 48 to 72 hours after the injection. Further evaluation and additional tests may be needed for a definitive diagnosis. While the Mantoux test has its limitations, it remains an important tool in the fight against TB, particularly in high-risk populations.