How does a skin prick test work?
The gold standard for allergy testing is as simple as pricking your skin, inserting a small amount of a substance, and waiting to see what happens. If you’re allergic to the substance, a reddish, elevated bump with a red ring around it will appear. This bump may be severely itchy.
An allergen is any substance that elicits an allergic reaction. When an allergen is inserted under a layer of your skin in a skin prick test, your immune system kicks into overdrive. It sends out antibodies to defend against what it believes to be a harmful substance.
When the allergen binds to a specific type of antibody, this triggers the release of chemicals, such as histamine. Histamine contributes to an allergic reaction. During this reaction, certain things happen in your body:
- Your blood vessels widen and become more porous.
- Fluid escapes from your blood vessels, which causes redness and swelling.
- Your body produces more mucus, which leads to congestion, runny nose, and teary eyes.
- Your nerve endings are stimulated, which causes itching, rash, or hives.
- Your stomach produces more acid.
In more severe cases, two other things may happen:
- Your blood pressure drops because of widened blood vessels.
- Your airways swell and your bronchial tubes constrict, making it hard to breathe.
Before you’re given a skin prick test, your doctor will talk with you. You’ll discuss your health history, your symptoms, and the types of triggers that seem to set off your allergies. Your doctor will use this information to determine which allergens to use in testing. Your doctor may test you for as few as three or four substances or as many as 40.
The test is usually performed on the inside of your arm or on your back. Typically, a nurse administers the test, and then your doctor reviews your reactions. Testing and interpreting the results usually takes less than an hour but the time depends on the number of allergens being tested.
Your main task prior to testing is to provide details about your allergies, such as when and where your allergies act up and how your body responds.
You shouldn’t take antihistamines before the test. Let your allergist know which antihistamine you usually take. Depending on how it works, you may need to be off it for over a week’s time. This includes cold or allergy medications containing an antihistamine combined with other substances.
Other medications may alter the result of the skin prick test as well, so you’ll need to discuss this with the allergist in case you need to hold off taking them for a time leading up to testing. On the day of testing, don’t use lotion or perfume on the area of skin where the test will be performed.
You might test positive for an allergen but never show symptoms of that allergy. You may also get a false positive or a false negative. A false negative can be dangerous because it doesn’t indicate the substance you are allergic to, and you won’t know to avoid it. It’s still a good idea to get tested because identifying the substances that do trigger your allergies enables you to work with your doctor to develop a treatment plan to ease your symptoms.
To perform the test:
- The area of your skin to be tested will be cleaned with alcohol.
- The nurse will make a series of marks on your skin. These marks will be used to keep track of the different allergens and how your skin reacts to them.
- A small drop of each allergen will be placed on your skin.
- The nurse will lightly prick the surface of your skin under each drop so a small amount of the allergen will seep into the skin. The procedure isn’t usually painful but some people find it slightly irritating.
- After this part of the test is complete, you’ll wait for any reactions, which usually peak within 15 to 20 minutes. If you’re allergic to a substance, you’ll develop a red, itchy bump. The area where the allergen was placed will look like a mosquito bite surrounded by a red ring.
- Your reactions will be evaluated and measured. The bumps from the skin reaction usually disappears within a few hours.
Skin prick testing can be performed on people of all ages, even infants if they’re older than 6 months. It’s widely used and safe in most cases. Rarely, a skin prick test can trigger a more severe type of allergic reaction. This is more likely to occur in people with a history of severe reactions. It’s also more common with food allergies. Your doctor will be prepared to recognize and treat these reactions.