What is the Mantoux Tuberculin Skin Test?
The
Mantoux tuberculin skin test (TST) is a diagnostic tool used to determine if a person has been exposed to the bacterium
Mycobacterium tuberculosis, which causes tuberculosis (TB). It involves the intradermal injection of a purified protein derivative (PPD) into the forearm and observing the skin's reaction after 48-72 hours.
How Does the Test Work?
The test measures the immune response caused by the
PPD. When injected, the PPD stimulates a delayed-type hypersensitivity reaction in individuals who have been exposed to TB bacteria. This reaction results in localized swelling and induration at the site of injection, which is measured in millimeters to determine the test's outcome.
Who Should Get Tested?
The test is recommended for individuals at higher risk of TB exposure, including healthcare workers, those living in close quarters like prisons, and people from countries with a high prevalence of TB. It is also used for people with weakened immune systems, such as those with
HIV or undergoing immunosuppressive therapy.
- 0-4 mm: Generally considered negative.
- 5 mm or greater: Positive for individuals with HIV, recent TB contacts, or organ transplant recipients.
- 10 mm or greater: Positive for recent immigrants from high-prevalence countries, IV drug users, or residents of high-risk settings.
- 15 mm or greater: Positive for individuals with no known risk factors.
What Are the Limitations of the Test?
The Mantoux test has limitations, including false positives and false negatives. A false positive may occur due to previous
BCG vaccination or exposure to non-tuberculous mycobacteria. False negatives can result from recent TB infection, very old TB infection, or severely weakened immune systems.
How Does It Compare to Other Tests?
The
Interferon-Gamma Release Assays (IGRAs) are blood tests also used to detect TB infection. Unlike the TST, IGRAs are not affected by previous BCG vaccination and are preferred in cases where BCG interference is a concern. However, the Mantoux test remains widely used due to its simplicity and cost-effectiveness.
Are There Any Risks?
The Mantoux test is generally safe, with minimal risks. Some individuals may experience mild itching, swelling, or redness at the injection site. Severe allergic reactions are rare. It's important for individuals to inform healthcare providers of any previous severe reactions to PPD.What Should Be Done After a Positive Test?
A positive test indicates that the person has been exposed to TB bacteria. It does not confirm active disease, so further evaluation with a chest X-ray and possibly a sputum test is necessary to determine if the person has active or latent TB infection. Treatment decisions are based on these follow-up evaluations.Conclusion
The Mantoux tuberculin skin test plays a crucial role in the control of
tuberculosis, particularly in identifying individuals who may benefit from preventive therapy. Despite its limitations, it remains an essential tool in infectious disease diagnostics, especially in resource-limited settings. Understanding its application, interpretation, and limitations is vital for healthcare providers managing at-risk populations.
