Skin test anergy

How the Test is Performed:
The 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, the skin should appear red or irritated where the antigen was injected. The size and degree of irritation will determine whether you are allergic to the antigen. A second visit is required to have the results confirmed. You may be given washing instructions so that the test will be accurate.

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

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:



How it Feels:
When the antigen is injected, there is a slight stinging sensation where the needle is inserted. After the reaction begins, the area will probably itch or burn.

Risks:
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 .

Why this Test is Performed:
This test is very helpful in determining if a person's immune system is functioning properly. A skin test can indicate no exposure to a particular antigen , when actually the immune system is unable to react to any antigen.

Normal Values:
The injection site will show a greater than 5 mm inflammation zone at the test site of the test antigen .

There should be less than a 5 mm inflammation zone of the disease antigen.

Abnormal Results:
An inflammation zone of less than 5 mm indicates the immune system may be unable to react to any antigen . Other tests may be performed to determine if the person has been exposed to an antigen.

Cost:
The estimated cost is $25 to $30 per test.
A battery of tests may cost from $100 to $200.
Public health laboratories may charge as little as $5 to $20 per test.

Special Considerations:
Some skin tests are less accurate than others. 20 to 30% of patients with tuberculosis have negative tuberculin skin 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

mumps , Candida, or trichophytin is injected just under the skin (intradermal) to determine if a patient's immune system is able to react to antigens. This test is usually performed at the same time as another skin test. Learn to make informed health care decisions with the Medical Encyclopedia at drkoop.com.">