In a skin test anergy, a common antigen such as mumps, candida, or trichophytin is injected just under the skin. The purpose is to determine if a patient's immune system is reacting properly to antigens in general.

An anergy test is usually just a control for some other skin test that is being performed. Because the anergy test can show whether your immune system is functioning properly or not, it can indicate whether the results of the other skin test are reliable.

An antigen is injected just under the skin, usually on the forearm, so that a small lump pushes the skin up. The lump indicates that the antigen has been injected at the correct depth. The site is labeled, usually with an ink pen.

After 48 to 72 hours, if the immune system reacts to the antigen, the skin will appear red or irritated where the antigen was injected. The size and degree of irritation will determine whether you are responding to the antigen. A second visit after 48 to 72 hours is required to have the results confirmed. You may be given washing instructions so that the test will be accurate.

There is no special preparation. People with dermatitis or other skin irritations may have the test performed at different locations on the body.

When the antigen is injected, there is a slight stinging sensation where the needle is inserted. After the reaction begins, the area may itch or burn. Sometimes a strong reaction can turn into a large, red, irritated area.

This test serves as a control for some other skin test, such as a tuberculosis test, to make sure the other test is not giving a "false-negative" result.

For example, a patient might have a negative tuberculosis test. If he or she has a healthy immune system, a negative result generally means that person does not have tuberculosis. But if that person's immune system is suppressed, the negative test result may not be meaningful one way or the other -- because the patient's immune system is not reacting properly to any antigen at all. The idea of an anergy test is to help identify a false-negative result.

If your immune system is functioning properly, the injection site of the anergy test will show a greater than 5 mm inflammation zone.

An inflammation zone of less than 5 mm indicates the immune system may be unable to react to any antigen. Other tests, such as serologies, may be performed to determine if the person has been exposed to an antigen, to confirm these results.

A very small amount of the antigen is injected, so the risks are minimal. Typical reactions include itching and hives.

A patient who is hypersensitive to the antigen may experience respiratory distress or other signs of anaphylactic shock.

Some skin tests are less accurate than others. A significant number of patients with active tuberculosis have negative tuberculin skin tests (called PPD or Tine tests). Tetanus immunization (vaccine) can cause false positive results for diphtheria (Schick test). Tests for food allergies may be less accurate than disease antigen testing.

Other factors that can affect the test results include:

• Where on the body the test is performed (the arm reacts differently than the back)
• The time of day (morning tests react differently than afternoon tests)
• Proximity (closeness) to other tests
• Antihistamines can cause a false negative test (negative results when the antigen should show reaction)
• Old antigens are used