What is a tuberculin skin test?

This is a skin test done with tuberculin which contains protein from TB bacilli. Usually PPD (i.e. Purified Protein Derivative) is the form of tuberculin which is used. It does not contain live TB bacilli. The most accurate method of tuberculin skin testing is the Mantoux test, when a small amount of PPD is injected into the skin (intradermally). If a large area of swelling (induration) develops after 2 to 3 days at the injection site, the test is said to be positive. A positive skin test indicates that the child is infected, or has previously been infected with the TB bacillus, or been given BCG. As a result the child has now developed a sensitivity (an ‘allergy’) to the PPD.

The Tine test is also used but is not as accurate as a Mantoux test.

The induration at the site of the skin test is due to delayed sensitivity to the PPD, which develops at the time of the primary infection or the BCG immunisation. This indicates a good cellular immune response. The effect of BCG on the tuberculin skin test, however, does not last more than 2 years. Other signs of more marked sensitivity to Mycobacterium tuberculosis include phlyctenular conjunctivitis and erythema nodosum.

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