Skin testing (illustrated below) is the most common type of allergy test. Skin tests cover major categories, but cannot classify all allergies. Skin tests are rapid, simple, and relatively safe. In some extremely allergic patients who have severe reactions called anaphylactic reactions, skin testing cannot be used because it could evoke a dangerous reaction. Skin testing also cannot be done on patients with extensive eczema. There are two forms: percutaneous and intradermal.

In percutaneous or prick testing, allergen solutions are placed on the skin, and the skin is then pricked, allowing the allergen to enter the skin and become exposed to mast cells. Intradermal testing involves directly injecting allergen solutions into the skin.

If the skin test is positive, a reddened, swollen spot (wheal) develops at the injection site. This implies that the patient has the IgE antibody that releases histamine to cause redness and itching. This does not mean the substance actually causes the patient's symptoms. A positive test only shows that you are sensitive to the allergen. It does not necessarily mean that you will have symptoms on exposure to that allergen, or that the allergen is a cause of your symptoms.

Risks: Skin testing does not usually require any aftercare. Intradermal testing may inadvertently result in the injection of the allergen into the circulation, with an increased risk of adverse reactions. A generalized redness and swelling may occur in the test area, but it will usually resolve within a day or two.

If a skin test cannot be done, we may use special blood tests, such as RAST. Radio-allergosorbent testing (RAST) is a blood test performed when the patient may be too sensitive to risk skin testing, or when medications or skin conditions prevent it.

The patient's blood serum is combined with allergen in a test tube to measure the presence of specific types of IgE in the blood.

These tests cost more than skin tests, and the results are not available immediately because the test is performed in the laboratory. As with skin testing, positive RAST tests do not by themselves necessarily make the final diagnosis.
Provocation testing is done to positively identify suspected airborne or food allergens after preliminary skin testing.

Food testing is tedious because full passage through the digestive system may take a day or more. It’s usually done by removing the suspect food from the diet for two weeks, followed by eating a single portion with follow-up monitoring. Gastrointestinal symptoms within 24 hours indicates a positive response. You will be asked to keep a diet diary to determine what foods might be causing your symptoms.

Risks: Exposure to new or unsuspected allergens in any test carries the risk of anaphylaxis (blood vessel dilation, swelling, and smooth muscle contraction, which can result in sharp blood pressure drops and difficulty breathing). Patients are monitored following allergy testing, so anaphylactic reaction is usually recognized and treated promptly. Occasionally, a delayed anaphylactic response can occur that will require immediate care. Proper patient education regarding how to recognize anaphylaxis is vital. Patients will be instructed in recognizing and treating their potential reactions.
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Skin testing
(Above) Allergen solutions are placed on the skin, and the skin is then pricked, allowing the allergen to enter the skin. After 20 minutes, the allergen solutions are wiped off, and each site is assigned a score.

(Below) allergen solutions are injected just under the skin. Each site is marked, precisely measured, and scored.