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The Mantoux test, also known as the tuberculin skin test (TST), is carried out to detect exposure to Mycobacterium Tuberculosis, the bacterium responsible for causing tuberculosis. This test is performed as a standard screening procedure in countries with high prevalence of tuberculosis, and as one of the diagnostic tests for suspected Tuberculosis infection. 

Why Do We Need To Do A Mantoux Test? 

In principle, a positive Mantoux test reflects a history of exposure to TB. Therefore, in many clinical settings, a Mantoux test is performed to diagnose TB in the following scenarios:

  1. On health workers who are constantly exposed to TB while treating patients. 
  2. During the assessment of family members or close contacts of a patient recently diagnosed with TB. 
  3. In People who test positive for HIV, as TB treatment or prevention will be required as part of the holistic treatment strategy.
  4. In people with symptoms highly suggestive of TB such as long standing cough, weight loss, fever and night sweats. 

How Is A Mantoux Test Performed?

It is performed by injecting a small amount of the killed and processed bacterium known as purified protein derivative ( PPD) just underneath the top later of the skin (intra-dermally). In those who have had prior exposure the bacterium, their immune cells are activated and they induce the characteristic skin lesions seen. Standard testing involves the injection of 5 units of PPD intra-dermally on the left forearm. After 2-3 days (48-72 hours), a thickened, red and raised skin is visible and palpable over the injection site. This known as an induration. During the interval period before the appearance of induration, sensations of itching may be experienced at the injection site, but scratching must be avoided as much as possible as this may result in a false positive result (Long, Holmes, & Ismeurt, 1993).

What Is A Positive Mantoux Test And What Does It Indicate? 

A positive Mantoux test is signified by a skin induration measuring 5-15mm, 72 hours after the test is carried out (Rose, Schechter, & Adler, 1995) (Nayak & Acharjya, 2012). In the absence of any previous exposure to M.tuberculosis, little or skin induration is observed, and this is indicative of a negative test. A positive test indicates past exposure to TB, but does not signify active disease. Due to the critical importance of a truly positive Mantoux test, intense training is provided to health workers to ensure that positive tests are identified, while those who test negative are truly negative. A false positive Mantoux test may result from: 

  • Exposure to harmless mycobaterial species in the environment.
  • Improper technique of test administration or result interpretation. 

Similarly, a false negative Mantoux test may arise in the following settings:

  • Improper technique of test administration or result interpretation.
  • Following very recent Tb infection, or after several decades of infection.
  • In patients with very advanced TB infection. 
  • Testing infants. 

What Risks Are Associated With A Mantoux Test? 

Generally, there are little or no risks associated with the Mantoux test. It is important to remember that TB infection cannot occur from the injection of PPD. However, it is very risky in patients with a history of allergic reaction following a Mantoux test. 

What Next After A Positive Mantoux Test? 

Further tests are performed in order to delineate TB infection status, the extent of local lung involvement, dissemination to distant body sites, and thereafter, treatment requirements. 

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