Michael A. Gardam MSc, MD, CM, FRCPC
Medical Director, Tuberculosis Clinic
Associate Hospital Epidemiologist
University Health Network
What is a Skin Test and How is it Administered?
Tuberculin skin testing is the most established method of diagnosing tuberculosis infection, that is both active disease and asymptomatic latent infection. Different skin testing techniques have been developed over the past 70 years. The Mantoux test, however, is the standard procedure in North America. The Mantoux test involves the intradermal injection of 0.1 ml of purified protein derivative (PPD--a precipitate prepared from filtered heat-sterilized cultures of Mycobacterium tuberculosis). The only absolute contraindication to administering the test is a history of anaphylaxis induced by any of the components. Those with a history of BCG vaccination may be skin tested.
The test is usually administered in an area that is free of blood vessels, hair or edema, on the flexor surface of the forearm, but it may also be administered on the upper chest or back. The needle should be inserted just under the skin with the bevel facing up until the bevel is fully inserted. A bleb should be raised when the PPD is injected. If this is not accomplished, or the PPD leaks out onto the skin, the test should be readministered in a different site. The test must be read at 48 to 72 hours by a trained healthcare professional.