The substance being tested is injected into the intradermal layer of skin with a tuberculin syringe fitted with a short-bevel, 26- or 27-gauge needle. A positive reaction produces a red, inflamed area at the site of the injection within a given time period (e.g., 48–72 hours for the Mantoux test for TB).

Skin tests that indicate hypersensitivity to a toxin from an infection-producing agent may also signal immunity to the infectious agent. Positive reactions may also indicate an active or inactive phase of the infection under study. Skin tests can be categorized according to their nature and purpose as follows:

1. Tests to reveal a present or past exposure to the infectious agent, for example, tuberculin test (positive reaction indicates presence of active or inactive TB)

2. Tests to show sensitivity to materials toward which a person may react in an exaggerated manner; for example, allergenic extracts such as house dust and pollen (positive reaction indicates sensitivity to allergen extracts)

3. Tests to detect impaired cellular immunity. Intradermal skin testing with several common antigenic microbial substances (e.g., purified protein derivative [PPD] tuberculin, mumps virus, C. albicans, streptokinase-streptodornase) can determine whether immune function is normal. This would be important in treating leukemia and cancer with chemotherapy. (Negative reaction to any intradermal antigen indicates impaired immunity.)

1. Follow the manufacturer’s instructions for the diagnostic skin tests. Most are prepackaged as sterile kits.

2. Inject the ordered amount of the test material intradermally on the volar aspect of the forearm.

3. A positive reaction is generally manifested by redness or swelling more than 1 cm in diameter at the injection site. A central area of necrosis is a highly significant finding.