Mantoux test or MT test

Mantoux test or MT test

The Mantoux test or Mendel-Mantoux test (also known as the Mantoux screening test, tuberculin sensitivity test, Pirquet test, or PPD test for purified protein derivative) is a screening tool for tuberculosis (TB). It is one of the major tuberculin skin tests used around the world, largely replacing multiple-puncture tests such as the Tine test.   The Heaf test  was used until 2005 in the UK, when it was replaced by the Mantoux test. It is endorsed by the American Thoracic Society and Centers for Disease Control and Prevention. It was also used in the USSR and is now prevalent in most of the former Soviet states.

 

Procedure:  A standard dose is 5 tuberculin units (TU – 0.1 ml) (The standard Mantoux test in the UK consists of an intradermal injection of 2 TU of Statens Serum Institute (SSI) tuberculin RT23 in 0.1 ml solution for injection.) is injected intradermally (between the layers of dermis) and read 48 to 72 hours later. This intradermal injection is termed the Mantoux technique. 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, or had a recent tuberculin skin test (within one year), another skin test should be used.

Classification 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 TB exposure.

  • 5 mm or more is positive in
    • An HIV-positive person
    • Persons with recent contacts with a TB patient
    • Persons with nodular or fibrotic changes on chest X-ray consistent with old healed TB
    • Patients with organ transplants, and other immunosuppressed patients
  • 10 mm or more is positive in
    • Recent arrivals (less than five 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 four years of age, or children and adolescents exposed to adults in high-risk categories
  • 15 mm or more is positive in
    • Persons with no known risk factors for TB

(Note: Targeted skin testing programs should only be conducted among high-risk groups)

A tuberculin test conversion is defined as an increase of 10 mm or more within a two-year period, regardless of age. Alternate criteria include increases of 6, 12, 15 or 18 mm.

False positive result: TST (Tuberculin skin test) positive is measured by size of induration. The measurement of the induration is determined to be positive depending on risk factors. For example, a low-risk patient must have a larger induration for a positive than a high-risk patient. High-risk groups include recent contact, HIV, chest radiograph with fibrotic changes, organ transplant recipient, and immunosuppression.

According to the Ohio Department of Health and US Department of Health, the Bacillus Calmette–Guérin (BCG) vaccine does not protect against TB infection. It does, though, give 80% of children protection against Tuberculous meningitis and miliary tuberculosis. Therefore, a positive TST/PPD in a person who received BCG vaccine is interpreted as latent TB infection (LTBI). Due to the test’s low specificity, most positive reactions in low-risk individuals are false positives. 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 positives can also occur when the injected area is touched, causing swelling and itching.

Another source of false positive results can be from allergic reaction or hypersensitivity. Although rare, about 0.08 reported reactions per million doses of tuberculin, these reactions can be dangerous and precautions should be taken by having epinephrin available.

 

False negative result: The result of PPD or tuberculin test is suppressed by the following conditions:

  • Infectious mononucleosis
  • Live virus vaccine- The test should not be carried out within 3 weeks of live virus vaccination and should be rearranged.
  • Sarcoidosis
  • Hodgkin’s disease
  • Corticosteroid therapy/Steroid use
  • Malnutrition
  • Immunologically compromised- Those on immuno-suppressive treatment or 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. A false negative result may occur in a person who has been recently infected with TB, but whose immune system hasn’t yet reacted to the bacteria.

  • Upper respiratory virus infection.

In case a second tuberculin test is necessary it should be carried out in the other arm to avoid hypersensitising the skin.

 

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