Mantoux Test In Tuberculosis

The Mantoux test is very widely used test for tuberculosis. Its interpretation however complex. It is also called tuberculin test.

Tuberculin is a glycerol extract of the tubercle bacillus. Purified protein derivative (PPD) tuberculin is a precipitate of non-species-specific molecules obtained from filtrates of sterilized, concentrated cultures.

The test is named after Charles Mantoux, a French physician who developed on the work of Koch.

Procedure

A standard dose of 5 Tuberculin units (0.1 mL) is injected intradermally and read 48 to 72 hours later. A person who has been exposed to the bacteria is expected to mount an immune response in the skin containing the bacterial proteins.

The reaction is read by measuring the diameter of induration (palpable raised hardened area) across the forearm (perpendicular to the long axis) in millimeters. If there is no induration, the result should be recorded as “0 mm”.

Erythema (redness) should not be measured.

If a person has had a history of a positive tuberculin skin test, another skin test is not needed.

Interpretation of tuberculin reaction

The results of this test must be interpreted carefully. The person’s medical risk factors determine at which increment (5 mm, 10 mm, or 15 mm) of induration the result is considered positive.

A positive result indicates an exposure with mycobacterium tuberculosis.

Normally generalised interpretation of Mantoux test are

Induration 0-5 mm  – Negative

Induration 6-9 mm  – Weak positive

Induration > 10 mm – Positive

However depending upon risk factors the interpretation is considered positive as follows

Induration of 5 mm or more is positive in

• HIV-positive person
• Recent contacts of TB case
• Persons with nodular or fibrotic changes on chest x-ray consistent with old healed TB
• Patients with organ transplants and other immunosuppressed patients

Induration of 10 mm or more is positive in

• Recent arrivals (less than 5 years) from high-prevalence countries
• Injection drug users
• Residents and employees of high-risk congregate settings (e.g., prisons, nursing homes, hospitals, homeless shelters, etc.)
• Mycobacteriology lab personnel
• Persons with clinical conditions that place them at high risk (e.g., diabetes, prolonged corticosteroid therapy, leukemia, end-stage renal disease, chronic malabsorption syndromes, low body weight, etc)
• Children less than 4 years of age, or children and adolescents exposed to adults in high-risk categories

Induration of 15 mm or more is positive in

• Persons with no known risk factors for TB

False positive result

A false positive result may be caused by nontuberculous mycobacteria or previous administration of BCG vaccine. Prior vaccination with BCG may result in a false-positive result for many years afterwards.

False negative result

Those that are immunologically compromised, especially those with HIV and low CD4 T cell counts, frequently show negative results from the PPD test. This is because the immune system needs to be functional to mount a response to the protein derivative injected under the skin.

Patients with highly active TB infection may also show false negative results.

Anergy testing

In cases of anergy, a lack of reaction by the body’s defence mechanisms when it comes into contact with foreign substances, the tuberculin reaction will occur weakly, thus compromising the value of Mantoux testing. For example, anergy is present in AIDS, a disease which strongly depresses the immune system. Therefore, anergy testing is advised in cases where suspicion is warranted that it is present. However, routine anergy skin testing is not recommended.

This induration is a result of the hypersensitivity that the individual develops to the bacilli. Hence a positive Mantoux test only indicates that the individual got infected with Mycobacterium tuberculosis sometime during his life. It should not be misinterpreted as a sign of tuberculous disease and ATT should not be started in adults solely on the basis of a positive Mantoux test.

More than 50 percent of Indian adults are Mantoux positive, indicating a high prevalence of infection with Mycobacterium tuberculosis. 90 percent of those infected do not develop pulmonary TB in their lifetime. It is clear that a positive Mantoux test does not indicate and is not a sign of disease in adults.

A test is either positive or negative depending on whether the induration is less than or more than 10 mm in diameter. There is no added significance of a “strongly positive” reaction.