Types of Allergy Tests
Together, you and your doctor will determine which type of allergy testing is best for you. In the table below, read about some of the pros and cons of each type of allergy test. Get more information about each type of test on the following pages.
TEST TYPE PROS CONS Skin testing (scratch, intradermal)
- Immediate results
- Sensitive
- Inexpensive
- Multiple skin pricks/punctures
- Some medications can affect results
- Rare risk of possible life-threatening reaction (anaphylactic shock)
- Can't be used for people who have widespread skin problems or are taking certain medications
- No risk of anaphylactic reaction
- Good for children and people who have skin problems or who are taking some medications
- One needle stick to draw blood
- Delayed results
- More expensive
- Requires needle stick
- Immediate results
- Can be used when food or drugs that are possible allergens are needed
- Risk of possible life-threatening reaction (anaphylactic shock)
- Expensive
See the next page to learn more about allergy skin tests.
Allergy Skin Tests
What it is. Allergy skin testing is the easiest, fastest, most sensitive, and, in many cases, the most cost-effective way to test for allergies. There are 2 types of allergy skin tests: the scratch test and the intradermal test.
How an allergy scratch test is done. Scratch tests involve applying a drop of allergen, such as dust mites, pet allergens, or pollens and molds, to tiny pricks or punctures made on the skin. Depending on how many allergens to which the doctor thinks you're allergic, he or she may perform as many as 20 or more scratch tests.
How intradermal testing is done. Intradermal testing involves injecting small amounts of diluted allergens under the top layers of the skin. Depending on how many allergens to which the doctor suspects you're allergic, he or she may perform as many as 20 or more tests.
What the results mean. A positive reaction is called a wheal and flare response. Within 10 to 15 minutes of application, the result is itchy, red, swollen areas that look like mosquito bites. The greater your reaction - the redder and more swollen the area is - the more allergic you are to the substance and the more likely you are allergic to it in the real world. While the size of the wheal can give your doctor important diagnostic information, a positive reaction by itself doesn't mean that particular allergen is the cause of your symptoms. On the other hand, if no reaction appears, the test is negative, and you're not allergic to that specific substance.
The benefits. Allergy experts say that intradermal tests are more sensitive than scratch tests in identifying things you may be allergic to. However, intradermal tests are also more likely to result in false positives. That means you have a positive test reaction but you don't have allergy symptoms in everyday life. While the different types of allergy tests provide clues to your condition, your doctor's experience and expertise also come into play in interpreting the test results and how they apply to you.
The precautions. Allergy skin testing is a less expensive and more sensitive way to test for allergies. However, a very few people, such as those who have widespread skin conditions such as eczema or who take medications that interfere with skin testing, such as antihistamines, can't use it. Antihistamines, found in over-the-counter allergy medications as well as in prescription allergy drugs, can interfere with allergy skin testing. Many over-the-counter sleeping pills also contain an antihistamine. To tell, look for the name on the label. If you're taking an antihistamine, ask your doctor or pharmacist how long prior to allergy skin testing you should stop taking it.
Allergy Blood Test
What it is. Allergy blood testing is one way to test sensitivity to allergens. The most common type of allergy blood test is called a radioallergosorbent test, or RAST. In this test, levels of IgE antibodies are detected by a highly sensitive technique called radioimmunoassay, or enzyme-linked assay. In this technique, radioactive or enzyme markers make the antibodies easy to spot.
How it is done. For the RAST, a small sample of your blood serum, the fluid left after red and white cells are removed, is placed in a machine. The machine measures the presence and number of particular IgE antibodies. If the test shows you have a lot of ragweed pollen IgE antibodies, for instance, you're probably allergic to ragweed. It usually takes a few days to get the results, and it's not quite as accurate in diagnosing allergies as the scratch test.
Allergy Challenge Tests
What it is. The allergy challenge test is used only if testing for a specific type of allergy isn't available or if your doctor thinks you may be allergic to a food or medication.
How it is done. In the challenge test, you inhale or swallow a tiny amount of the suspicious substance, such as milk or an antibiotic. Then you wait for a reaction. If there's no reaction, the allergist may slowly increase the dose.
The precautions. Allergy challenge tests may involve severe allergic reactions. They should only be used when absolutely necessary. They must be performed by a doctor who is trained and equipped to handle severe allergic reactions.