Allergy Diagnosis and Testing
How Are Allergies Diagnosed?
First, your doctor will ask you questions about your health and symptoms. Make sure to tell your doctor if anyone in your family has allergies. If family members have allergies, your chances of having allergies increase. Your doctor uses the following information to make a diagnosis of allergy:
- Physical exam
- History of your symptoms and family history
- Allergy tests (not always needed)
Allergy tests can be done to help identify if you are allergic and what you are allergic to. Once allergies are identified, specific avoidance and treatment measures can be recommended. There are several types of allergy testing.
What types of allergy testing can be done?
Scratch Skin Testing
A reliable test for allergies is the scratch skin test. A small amount of each thing you may be allergic to (allergen) is placed on the skin - often the back. The skin is then pricked. If you are allergic to an allergen, you will get a bump and redness where the skin is pricked. After a short time, each skin test reaction is measured for swelling and redness. If there is a large enough skin reaction, it means that you may be allergic to the allergen placed at that site. The information from your prick skin test results and your history of symptoms will help your doctor to determine if you have an allergy.
Antihistamines can affect the skin test results. Your doctor may tell you to stop these medications for days to weeks before the testing is done. Other medicines can also affect the results and may need to be avoided. Ask your healthcare provider what medicines to avoid before your scratch skin tests are done.
Home Allergy Management for Everyone
Intradermal Skin Testing
Another form of skin testing for allergy is by intradermal skin testing. This method is not as reliable as prick skin testing. It is most often used when prick skin testing is negative and there is a strong suspicion of allergy from the history. A small amount of each thing you may be allergic to (allergen) is placed under the skin with a needle, usually on the arm. If you are allergic to an allergen, you will get a bump and redness where the needle has gone under the skin. After a short time, each skin test reaction is measured for swelling and redness. If there is a large enough skin reaction, it means that you may be allergic to the allergen placed at that site. The information from these test results and your history of symptoms will help your doctor to determine if you have an allergy.
Antihistamines and other medicines can also affect these skin test results. Ask your healthcare provider what medicines to avoid before your skin tests are done.
A blood test is another kind of test that can be done to help find out if you have allergies. There is some evidence that blood tests are not as sensitive as prick skin tests in determining allergies. However, a blood test may be done if you have skin problems or, if there is concern that someone will have a severe reaction to a skin test (this is very rare). There are many types of blood tests that can be used to detect allergies. The most common one is called RAST testing.
The radioallergosorbent (RAST) test, a blood test done in a laboratory, can be used instead of skin tests to detect allergies. The test is no more accurate than skin tests, but much more expensive. Still, the test is valuable in a few situations.
Some skin conditions (such as severe eczema or atopic dermatitis) might make skin testing impossible, or the physician might suspect that a patient is so sensitive to an allergen that a skin test could be dangerous. An advantage of the RAST test is that it's not affected by antihistamines or other medications that the patient may be taking. Variations of the RAST include FAST (fluoro-allergosorbent test) and MAST (multiple antigen simultaneous testing).
Positive skin or RAST tests help narrow the list of suspected allergens, but don't often point directly to the cause of symptoms. The patient's history is vital in determining which of the positive skin test or RAST results really contribute to allergy symptoms. A challenge test may also be needed. If the patient's allergic reactions haven't been severe, the physician might deliberately expose the patient to one or more substances that produced a positive skin or RAST test. This is particularly true with food allergies.
Patch Skin Testing
Patch skin testing may be used to find out if a rash is from direct contact with an allergen. Small amounts of allergens are placed on the skin, often the back. The skin is covered with a watertight bandage for several days. After several days the patch is removed and the skin reactions are measured to find out if you may have a contact allergy.