Skin Allergy Test

There are different types of allergy skin tests and each has a specific reason for use.

Skin allergy testing can be used to help diagnose substances that can trigger the immune system in the body to produce an allergic reaction.

Part of the body’s immune response to an allergen (a substance that triggers an allergic reaction) is for mast cells to release histamine. Mast cells are concentrated in the skin, respiratory tract (nose, mouth, lining of the lungs), the gastrointestinal tract, and in the blood. This is an immunoglobulin E (IgE) response.

Histamine causes inflammation and tissue swelling, and the symptoms seen in an allergic reaction: hives (urticaria), runny nose, swelling in the throat, wheezing, and vomiting and diarrhea.

The skin tests are used in combination with the clinical story and physical exam associated with the patients’ complaints, to make the diagnosis of an allergy to a specific allergen.

There are different types of allergy skin tests and each has a specific reason for use.

The skin prick test is usually used to test for allergies to foods, or airborne allergens like pollen, dust, dust mites, pet dander, and insect venom.

The intradermal test may be a second step test done if the skin prick test is negative for an allergen, but the health care provider is still concerned that an allergy exists.

Skin patch tests are usually done to diagnose allergies that cause contact dermatitis (derm=skin + itis=inflammation).

Skin prick tests turn positive almost immediately, while skin patch tests may take days for the allergy to show itself.

Skin allergy test vs. blood test for allergies

Skin tests tend to be more accurate in determining allergies and the results are known almost immediately (within 20-30 minutes). Blood tests are less reliable, but have a place in diagnosing allergies when skin testing cannot be done. Blood testing is advancing rapidly and is frequently now used in conjunction with skin testing or replacing skin testing depending on the patients circumstances and allergens tested for.

Some patients with rashes, or who are taking antihistamines, may not be candidates for skin testing.

Some patients may be so sensitive to an allergen that skin testing would not be safe because of the risk of a major allergic reaction.

The skin prick allergy test is relatively noninvasive and is well tolerated. A drop of the purified allergen is placed on the skin, usually a forearm, and a prick is made through the drop into the skin using a tiny needle. If an allergy is present, small red bumps (a red wheal) will occur within 15-20 minutes and then usually fade within an hour.

In addition to the allergen, two other skin pricks are done as controls, so the doctor can make certain that the patient’s body has normal reactions to allergens.

• Histamine: In most people, when histamine is injected under the skin, it will cause a local allergic reaction with red wheals developing at the injection site. If histamine does not cause the wheal, then the patient may not be able to mount any allergy and a negative test result for an allergen may not necessarily be believed.
• Glycerin or saline: These are inert substances and the body should not develop an allergic reaction to them. If wheals develop, then the skin may be hypersensitive, and a positive test result may lead to a false allergy diagnosis.

If the skin prick test is negative, meaning no skin reaction occurred, the decision may be made to perform an intradermal skin test.

For an intradermal skin test, a tiny amount of the allergen is injected just beneath the skin surface. Because more allergen is used, there is a possibility of an allergic reaction taking place, including a runny nose or skin hives. It is rare that anaphylaxis (a whole-body allergic reaction) would occur with swelling of the mouth and tongue, and loss of blood pressure.

Intradermal skin tests are not done for food allergies.

The skin prick and intradermal skin tests take only a few minutes to perform in the office.

With skin patch tests for contact allergies, many small patches, each containing a small amount of allergen, are placed on the back and left in place. The patient goes home to return in 48 hours and then again in 5-7 days. It may take that long for a delayed allergic reaction to show itself.

The skin patch test is most commonly used for suspected allergies to lates, metals (especially nickel), preservatives, dyes, and fragrances.

Skin allergy testing is positive if a wheal and flare develop, meaning there are red bumps with surrounding redness. The wheal is measured and needs to be at least 3 mm in diameter. The larger the wheal, the more sensitive the patient is to the specific allergen.

To confirm an allergy, the wheal needs to be positive and the controls need to react appropriately on the skin.

Skin Allergy Test Results Chart

Skin allergy tests are reasonably accurate, but need to be interpreted in association with the clinical condition.

Should a larger amount of allergen be injected, it may cause a positive test result, even when the patient is not allergic.

Some medications can interfere with skin allergy testing, including prescription and over the counter antihistamines, some heartburn medications, some asthma medications, and tricyclic antidepressants.

Skin allergy testing is a safe procedure, but needs to be performed by a physician trained in allergy and immunology. In rare cases, patients may develop a severe allergic reaction (anaphylaxis). This needs to be recognized and treated immediately.

These major allergic reactions occur (though rarely) with intradermal skin allergy testing.

Feeling sick after a skin allergy test

Should an allergy skin test cause symptoms, they are usually mild and last for a short period of time. Watery eyes or a stuffy, runny nose may last for a few minutes. Itchiness of the skin surrounding the skin prick or intradermal injection usually lasts less than an hour, but may persist for a few days.