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 method used to detect latent tuberculosis (TB) infection. In this test, a small amount of purified protein derivative (PPD)—a substance derived from the Mycobacterium tuberculosis bacterium—is injected just under the skin of the forearm. After 48 to 72 hours, a healthcare professional examines the injection site for a reaction, typically measured by the size of any raised, hardened area (induration).
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History of the Mantoux Test
The Mantoux test was developed in 1907 by French physician Charles Mantoux, building on earlier research by Robert Koch and Clemens von Pirquet. In 1890, Koch introduced tuberculin, a protein extract derived from Mycobacterium tuberculosis, initially intended as a treatment for tuberculosis. Although it proved ineffective therapeutically, it was later recognized for its diagnostic potential. In 1907, Mantoux refined this approach by developing the intradermal injection method, using a diluted form of tuberculin to test for TB infection. His method replaced earlier scarification techniques and allowed for more accurate and standardized interpretation of immune responses. Since then, the Mantoux test has become a cornerstone of tuberculosis screening and diagnosis, especially in regions where TB remains a public health concern.
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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 exposure to tuberculosis, vaccinations (especially the Bacillus Calmette-Guérin (BCG) vaccine), and any 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. This injection creates a small, raised bump (called a wheal) at the injection site.
The injection must be performed correctly, placing the PPD between the layers of skin (intradermal) rather than deeper (subcutaneous), since improper technique can lead to inaccurate results.
C. Waiting Period
The patient is instructed to return 48 to 72 hours later so the healthcare provider can evaluate the reaction at the injection site. This waiting period allows time for the immune system to respond to the tuberculin if the person has been exposed.
D. Reading the Results
At the follow-up visit, the healthcare provider examines the injection site for induration (a raised, firm area). The size of the induration, measured in millimeters with a ruler, is the key factor in interpreting the test. Redness or discoloration alone is not considered when reading the results.
Interpretation of the Mantoux Test Results
The Mantoux test results are interpreted based on the size of the induration (raised, hardened area) at the injection site and the individual’s risk factors for tuberculosis (TB). The threshold for a positive result varies depending on the 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 distinguish between latent TB infection (LTBI) and active TB disease. The size of the induration considered positive depends on the individual’s risk category:
✅ 5 mm or more: Considered positive in high-risk individuals, including:
➧ People with HIV/AIDS
➧ Recent close contacts of active TB cases
➧ Individuals with chest X-ray findings suggestive of prior TB
➧ Immunocompromised patients (e.g., organ transplant recipients, those on long-term corticosteroid therapy)
✅ 10 mm or more: Considered positive in people with moderate risk factors, such as:
➧ Recent immigrants from countries with high TB prevalence
➧ Injection drug users
➧ Healthcare workers and persons living or working in congregate settings (prisons, nursing homes, homeless shelters)
➧ Children under 4 years old or children exposed to high-risk adults
✅ 15 mm or more: Considered positive in individuals with no known risk factors for TB. This threshold applies to healthy persons with a low risk of TB exposure.
B. Negative Result
A negative Mantoux test generally indicates that the person has not been infected with TB. However, a negative result does not always rule out infection. Factors such as recent TB exposure (within 8–10 weeks), immunosuppression, or anergy (a weakened immune response) can cause false-negative results.
C. False-Positive and False-Negative Results
➧ False-Positive: Prior BCG vaccination may cause cross-reactivity leading to false-positive results.
➧ False-Negative: Conditions that suppress the immune system, such as HIV infection, chemotherapy, or certain medications, may cause false-negative results, even in individuals with active or latent TB infection.
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 at high risk of tuberculosis exposure who do not show symptoms of active disease. People with LTBI are infected with Mycobacterium tuberculosis but do not have active TB and are not contagious. Early detection of LTBI is crucial because it can progress to active TB, so identifying and treating these cases helps prevent outbreaks.
B. Contact Tracing
After a confirmed active TB case, close contacts of the infected individual are tested with the Mantoux test to determine if they have been exposed. Identifying and treating LTBI among contacts can reduce the risk of future active TB cases.
C. Healthcare and Occupational Screening
Healthcare workers, prison staff, and other individuals at increased risk of TB exposure routinely undergo Mantoux testing. This helps detect infections early and allows for preventive measures to minimize transmission.
Limitations and Considerations
While the Mantoux test is a valuable diagnostic tool for tuberculosis, it has several limitations:
A. BCG Vaccination and False Positives
The Bacillus Calmette-Guérin (BCG) vaccine, commonly administered in many countries to protect against TB, can cause false-positive Mantoux test results. This occurs because of cross-reactivity between the vaccine strain and the purified protein derivative (PPD), complicating interpretation in vaccinated individuals.
B. Immunosuppression and False Negatives
Individuals with weakened immune systems, such as those with HIV/AIDS, cancer patients undergoing chemotherapy, or those on immunosuppressive medications, may have a reduced immune response to the test. This can lead to false-negative results, even if the person has a TB infection.
C. Need for Follow-Up Tests
A positive Mantoux test does not confirm active TB disease. Additional diagnostic procedures, such as chest X-rays, sputum cultures, or interferon-gamma release assays (IGRAs), are required to differentiate latent infection from active tuberculosis and to confirm the diagnosis.
Significance of the Mantoux Test
The Mantoux test, a simple intradermal injection, holds substantial importance in the ongoing global fight against tuberculosis (TB). Despite the advent of more advanced diagnostic tools, this century-old test remains a cornerstone of TB control strategies. It plays a critical role in identifying individuals exposed to Mycobacterium tuberculosis and, importantly, in preventing progression to active, transmissible disease. Its value lies in detecting latent tuberculosis infection (LTBI), supporting targeted screening programs, and maintaining relevance in specific diagnostic contexts.
One of the most significant roles of the Mantoux test is screening for latent tuberculosis infection (LTBI). LTBI represents a silent reservoir of potential active TB cases. Individuals with LTBI harbor the TB bacteria without symptoms and are not contagious. However, without treatment, a substantial portion of these individuals will develop active TB, sometimes years after the initial infection. The Mantoux test detects an immune response to TB proteins, serving as a key indicator of prior exposure. This enables healthcare providers to identify candidates for preventive therapy, an essential component of TB elimination efforts aimed at interrupting transmission and reducing future active cases.
Additionally, the Mantoux test is invaluable for targeted screening in high-risk populations. Its simplicity and low cost make it ideal for large-scale screening initiatives. High-risk groups include close contacts of active TB patients, immunocompromised individuals (such as those with HIV/AIDS), healthcare workers, immigrants from high TB prevalence regions, and residents of congregate settings like prisons, homeless shelters, and long-term care facilities. Routine Mantoux testing in these groups allows for early detection, enabling prompt evaluation and management. This focused approach is vital to prevent outbreaks and protect vulnerable populations.
While not a definitive diagnostic tool for active TB, the Mantoux test remains an important part of the comprehensive diagnostic process. A positive Mantoux test in a symptomatic individual, alongside clinical signs such as persistent cough, fever, and weight loss, triggers further investigations, including chest X-rays and sputum testing to confirm or exclude active disease. In certain cases, especially in children where sputum collection is difficult, the Mantoux test can have greater diagnostic value in guiding clinical decisions.
Beyond clinical use, the Mantoux test holds epidemiological significance. It can assess TB infection prevalence within populations or communities. Monitoring Mantoux positivity rates over time provides insights into the effectiveness of TB control programs and helps identify areas needing intensified intervention.
However, it is important to recognize the test’s limitations. False-positive results can occur in individuals vaccinated with BCG or exposed to non-tuberculous mycobacteria. False-negative results may arise in immunocompromised individuals or in those with severe active TB. These factors necessitate cautious interpretation of results in the context of clinical findings and risk factors, often requiring confirmatory testing.
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.
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.(alert-success)
