The Mantoux test: What is it and Why is it done?
The Mantoux test, also known as the Tuberculin Skin Test (TST), is a diagnostic tool used to detect tuberculosis (TB) infection. It works by injecting a small amount of a weakened form of the TB bacteria, called tuberculin, into the skin of the forearm. The test measures the body's reaction to the injection and can help determine if a person has been exposed to TB.
History of the Mantoux Test
The origins of the Mantoux Test date back to the early 20th century. French physician Charles Mantoux, based on previous work by Robert Koch and Clemens von Pirquet, developed the test in 1907 as a means to diagnose tuberculosis. While Koch had introduced tuberculin—a protein derived from the TB bacterium—as a possible therapeutic agent, it was Mantoux who refined its use as a diagnostic tool by injecting a diluted form of tuberculin intradermally. The test has since become a cornerstone of TB diagnosis worldwide, particularly in regions where tuberculosis is endemic.
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How is the Mantoux Test Done?
The Mantoux Test is a simple, safe, and minimally invasive procedure that involves the following steps:
A. Preparation
Before administering the test, the healthcare provider takes the patient’s medical history, including any previous TB exposure, vaccinations (especially the Bacillus Calmette-Guérin, or BCG, vaccine), and immunocompromising conditions, as these factors can affect the test results.
B. Intradermal Injection
A standard dose of 0.1 mL of purified protein derivative (PPD), containing 5 tuberculin units (TU), is injected intradermally into the inner surface of the forearm. The injection creates a small, raised bump (wheal) at the injection site.
The injection must be performed correctly, ensuring that the PPD is placed between the layers of skin (intradermal), not deeper (subcutaneous). Incorrect techniques can lead to unreliable results.
C. Waiting Period
The patient is asked to return after 48 to 72 hours for the healthcare provider to evaluate the injection site. This waiting period allows enough time for the immune system to react, if necessary, to the tuberculin.
D. Reading the Results
At the follow-up visit, the healthcare provider examines the injection site for any induration (raised, firm area). The presence and size of the induration, not redness, are measured with a ruler in millimeters (mm).
Interpretation of the Mantoux Test Results
The Mantoux Test results are interpreted based on the size of the induration and the patient's risk factors for TB. The threshold for a positive result varies depending on the individual’s likelihood of TB exposure or infection.
A. Positive Result
A positive Mantoux Test indicates that the person has been infected with Mycobacterium tuberculosis at some point. However, it does not differentiate between latent TB infection (LTBI) and active TB disease. The interpretation of a positive result depends on the size of the induration:
1.) 5 mm or More: Considered positive in high-risk individuals, including:
- People with HIV/AIDS
- Those who have had recent contact with an active TB patient
- Individuals with chest X-ray findings suggestive of previous TB
- Immunocompromised patients, such as those on organ transplants or long-term corticosteroid therapy
2.) 10 mm or More: Considered positive in people with moderate risk, such as:
- Recent immigrants from high-prevalence TB countries
- Injection drug users
- Healthcare workers and those living in congregate settings like prisons, nursing homes, or homeless shelters
- Children under 4 years of age or children exposed to high-risk adults
3.) 15 mm or More: Considered positive in individuals with no known risk factors for TB. This threshold applies to healthy people with a low risk of exposure to TB.
B. Negative Result
A negative result typically indicates that the person has not been infected with TB. However, in some cases, a negative result may not rule out infection. Factors like recent TB exposure, weakened immune systems, or anergy (lack of immune response) can result in a false negative.
C. False-Positive and False-Negative Results
False-Positive: Individuals who have received the BCG vaccine may show a false-positive result due to cross-reactivity between the PPD used in the test and the vaccine.
False-Negative: Certain conditions, such as immunosuppression (e.g., HIV, chemotherapy, or certain medications), can cause false-negative results, even in individuals with active or latent TB.
Applications of the Mantoux Test
The Mantoux Test is widely used for various clinical and public health purposes:
A. Screening for Latent TB Infection (LTBI)
The test is commonly used to screen individuals who are at high risk of TB exposure but do not show symptoms of active disease. People with LTBI are infected with M. tuberculosis but do not have active TB and cannot spread the disease. However, LTBI can progress to active TB, making early detection critical for preventing TB outbreaks.
B. Contact Tracing
After a confirmed case of active TB, close contacts of the infected individual are tested using the Mantoux Test to determine whether they have been exposed. Identifying and treating LTBI in these contacts can prevent future cases of active TB.
C. Healthcare and Occupational Screening
Healthcare workers, prison staff, and other individuals at high risk of TB exposure often undergo routine Mantoux testing. This ensures that potential infections are identified early and appropriate measures can be taken to prevent transmission.
Limitations and Considerations
While the Mantoux Test is a valuable tool for diagnosing TB, it has some limitations:
A. BCG Vaccination and False Positives
The BCG vaccine, widely used in many countries to protect against TB, can cause false-positive Mantoux Test results. This makes it challenging to interpret the test in individuals who have been vaccinated, as they may react to the PPD even without TB infection.
B. Immunosuppression and False Negatives
Individuals with weakened immune systems, such as those with HIV, cancer patients undergoing chemotherapy, or people on immunosuppressive drugs, may have a diminished reaction to the PPD. This can result in false-negative results, even in individuals who have TB.
C. Need for Follow-Up Tests
A positive Mantoux Test result does not confirm active TB disease. Further diagnostic tests, such as a chest X-ray, sputum culture, or interferon-gamma release assays (IGRAs), are needed to differentiate between latent and active TB and confirm the diagnosis.
Significance of the Mantoux Test
The Mantoux Test remains an essential tool in the global fight against tuberculosis. Its simplicity, affordability, and effectiveness in detecting latent TB infections have made it a cornerstone of TB control programs around the world.
A. Public Health Implications
Tuberculosis remains one of the leading causes of death worldwide, particularly in developing countries. Identifying and treating LTBI through the Mantoux Test can significantly reduce the incidence of active TB, which is crucial for curbing the spread of the disease.
B. Global TB Eradication Goals
The World Health Organization (WHO) and other public health organizations have set ambitious targets for reducing TB incidence and mortality. The Mantoux Test, along with newer diagnostic tools, plays a critical role in achieving these goals by identifying individuals with latent infections who may be at risk of developing active TB.
Summary
The Mantoux test is an important tool in the early detection of TB infection and is recommended for people who are at high risk of exposure to the bacteria. This includes healthcare workers, people with weakened immune systems, and those who have had close contact with someone who has active TB disease.
The Mantoux test is a simple and reliable method for detecting TB infection. It is important to seek medical attention if you suspect you have been exposed to TB, as early detection and treatment can prevent the development of serious complications. If you have a positive Mantoux test result, it is important to undergo further tests to determine if you have active TB disease and to receive appropriate treatment if necessary.
