About the Test
Purpose of the test
Allergy skin tests are used to determine what substances are causing a patient’s allergic reaction. Identifying allergens is an important part of guiding treatment for the symptoms of an allergic reaction because similar symptoms may be caused by other problems such as infections or reactions to medication. Some common allergens are:
- Pollen
- Pet and animal dander
- Mold
- Dust
- Food
- Insect venom
- Medications
- Household chemicals
There are three main types of allergy skin tests: prick tests, intradermal tests, and patch tests. All three use exposure to suspected allergens to attempt to induce an allergic reaction. Doctors use a patient’s medical history and geographic location to help decide which allergens will be tested.
Prick tests can detect most allergens and are the most common allergy skin test.
Intradermal tests are used for many of the same suspected allergies as prick tests. However, food allergies are not tested using this method because of a risk of a severe reaction and a high rate of inaccurate results.
Patch tests diagnose allergic reactions to allergens that come into contact with the skin.
What does the test measure?
All three types of allergy skin tests measure how much certain immune system cells react to specific allergens.
Prick and intradermal tests stimulate a response in cells within the skin called mast cells. In patients with an allergy, exposure to an allergen activates certain mast cells and produces a hive or small bump called a wheal.
Patch tests focus on the reactions of another type of cell called T cells. These cells are typically responsible for producing a rash at the test site in patients with an allergy, a condition called allergic contact dermatitis.
When should I get Allergy Skin Testing?
Your doctor may order allergy testing when they believe you to be allergic to one or more substances. This is indicated by your medical history along with the following signs and symptoms observed during a physical exam:
- Hay fever or a runny nose
- Red, watery eyes
- Itchy eyes, nose, or throat
- Hives or skin rashes
- Asthma
- Chest congestion, coughing, or wheezing
- Vomiting and diarrhea
- Severe reactions to insect stings
Finding a Allergy Skin Test
How to get tested
Allergy skin tests are primarily done by allergists, who are physicians that have specialized training in identifying and managing allergies and other immune disorders. Testing is performed in an allergist’s office after a physical exam and discussion of your history of symptoms.
Dermatologists, physicians who specialize in diseases of the skin, may perform patch tests to diagnose allergic contact dermatitis.
Can I take the test at home?
Allergy skin testing cannot be done at home because there is a chance of rare but serious allergic reactions. Testing should be performed by health care providers who are able to treat allergic reactions and who are trained to read the results of the test accurately. Allergy tests that are done at home are usually blood tests.
How much does the test cost?
The cost of allergy skin testing can include fees for office visits and testing supplies. Prick testing and intradermal testing are performed in a single visit. Patch testing requires several visits over a period of four to seven days. Each of these visits may have a charge.
Most insurance plans cover allergy testing, but you may have a copay or deductible. Check with your doctor and insurance company for your specific costs.
Taking an Allergy Skin Test
Allergy skin testing does not require the collection of specimens such as a blood or a tissue sample. Instead, samples of potential allergens are placed under or onto the skin to judge if there is an immune reaction. Tests are typically performed on the forearm, upper arm, or back. Your doctor will determine which option is the best for you.
Before the test
Medications such as antihistamines, corticosteroids, and some antidepressants can impact the results of allergy skin testing. You may be asked to stop taking these for up to a week before the test. Be sure to discuss all of your medications with your doctor to ensure accurate test results.
For patch tests, you should avoid heavy sun exposure and tanning for a minimum of two weeks prior to the test. Depending on the site where the test will be conducted, you may be asked to clip body hair a few days before the test.
During the test
What you can expect during the test will vary depending on the kind of test you are having.
In the prick test, small amounts of the suspected allergens are placed on the skin of the forearm, upper arm, or back. These spots are then pricked with an instrument so the allergen goes just under the skin.
In order to help interpret your reaction to suspected allergens, additional substances are placed on the skin to visualize how a positive and negative reaction may look. A drop of a solution with no allergens is used to show how a negative reaction will appear on the skin; a drop of solution with a histamine solution that commonly causes an immune response is used to show a positive reaction.
After 15 to 20 minutes, your doctor will check the testing site and compare any reactions against the control group. Positive reactions exhibit a wheal, or small bump, surrounded by a flare, which is a well-defined red area. Your doctor may measure the wheals because they should be 3 to 5 mm larger than the control to be considered positive.
The test may involve a small amount of pain when the skin is pricked. The most common side effect during the test is red, itchy skin. It is important that you do not touch the test sites as this can impact the results of the test.
There is a small chance of a serious and potentially life-threatening side effect called anaphylaxis, which is a severe whole-body reaction to an allergen. Anaphylaxis requires immediate medical attention, which is why it is important that testing be performed under medical supervision.
In the intradermal test, very small amounts of allergens are injected directly under the skin rather than being placed on it and then pricked. The testing site is checked after 15 to 20 minutes for any reactions such as redness or swelling.
The immediate side effects of intradermal tests are very similar to prick tests, including some pain at the time of injection and redness and itching during the test. While it is still a rare occurrence, the risk of anaphylaxis is greater with intradermal testing because a higher concentration of allergy substances is used.
In the patch test, patches with a small amount of the suspected allergens are taped on the skin of the back or upper arms for 48 hours. At the second visit, the patches will be removed and the test sites checked for any reaction, such as a red or itchy rash. Because it can take several days for reactions to develop, a third visit usually occurs at 72 to 96 hours after the application of the patches.
During the test, you will be asked to avoid getting the testing area wet, including through bathing or sweating. The primary side effect of patch tests is itching and irritation at the test sites which typically goes away when the patches are taken off. However, there is a small risk of dermatitis and an increase in your body’s allergic response to the allergens being tested.
After the test
There are no restrictions once testing is complete. However, you may have some irritated, itchy skin as a side effect.
Some people have delayed reactions that begin one to two hours after testing. Symptoms include swelling, warmth, redness, and itching at the test site and usually end in 24 to 48 hours. Inform your doctor if any of these delayed reactions occur.
Allergy Skin Test Results
Receiving test results
Prick and intradermal tests are read 15 to 20 minutes after the tests are administered, and results can be available within an hour. Your doctor will typically conduct the test and discuss test results at the same appointment.
Results from patch testing take longer to receive than the prick and intradermal tests. This is because the testing sites are evaluated several times between 48 and 96 hours from when the test is administered. Depending on the allergens being tested, additional evaluations may be needed for up to seven days.
Interpreting test results
Allergy skin tests results are typically reported as positive or negative.
If there is an immune reaction to a potential allergen, that is considered a positive result. If not, then the test result is negative, which may indicate the patient does not have an allergy to that substance. However, you may not receive a formal report with your results as testing cannot be used on its own to positively diagnose an allergy.
Skin test results must be evaluated alongside your medical history. Your doctor will discuss the results of your test with you as well as potential treatments.
Treatments for allergies can include:
- Lifestyle changes to avoid allergens
- Medications, such as antihistamines, corticosteroids, and decongestants
- Immunotherapy, a treatment that works to adjust your immune system to respond more effectively
Are test results accurate?
Skin tests are generally accurate and are the most common and reliable type of allergy testing. However, no test is perfect, and false negative and false positive test results do occur.
False negative test results occur when the test results are negative although the patient does in fact have an allergy.
False positive test results occur when a test result is positive even though the patient does not have an allergy. These results can occur because large enough amounts of an allergen may cause a reaction even in those who do not have an allergy.
Medications, UV exposure, and contact with the testing area can impact the validity of the test results, so it is important to follow all of your doctor’s instructions prior to and during testing.
Do I need follow-up tests?
Follow-up tests are not usually needed. However, there are some instances when doctors may order additional testing, such as:
- If prick testing is negative but your doctor believes you may have an allergy, they may order an intradermal test to confirm the prick test results.
- If standard patch testing does not determine the source of allergic contact dermatitis, you may be asked to repeat the test using your own personal products that may be causing allergy symptoms.
Questions for your doctor about test results
The following questions may be useful when discussing allergy skin test results with your doctor:
- What, if anything, am I allergic to?
- If there are positive test results, how can I limit exposure to those allergens?
- What is the best treatment plan for my allergies?
- Do I need any follow-up tests?
How is allergy skin testing different from allergy blood testing?
There are different kinds of allergy tests that can be used to detect allergies, including allergy skin testing and allergy blood testing. Allergy blood tests look for certain antibodies and cells that indicate the likelihood of an allergy. Unlike skin testing, allergy blood tests require a sample of blood to be collected using a needle. Because allergy blood tests take additional time to process in a laboratory, the results of these tests may take longer to come in.
Doctors may use blood testing instead of skin testing:
- When skin tests could be unsafe because the risk of severe reaction is higher
- When patients take medications that may interfere with getting accurate results
- When finding a location for skin testing is difficult on account of skin conditions
