Skin Prick Tests to Diagnose Food Allergies

Multiple prick tests performed at once can help identify potential allergens.

allergy prick test
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A prick test is often used by allergists when a patient has clear allergy symptoms but is not certain which food is causing those symptoms (or whether the symptoms are caused by a food at all). They are often used when patients have hives, eczema, or hay fever symptoms.

Prick tests have a number of advantages. They're among the quickest allergy tests available — you can perform them and get results in about 20 to 30 minutes. They also are reasonably inexpensive. They allow allergists to test a number of potential allergens at one time, as well. Finally, they are usually not painful, even though they involve a series of pricks on your skin.

They do have some disadvantages, however.

Prick tests are considered less sensitive than some other allergy tests, and a negative prick test may be followed up with other, more sensitive tests if a practitioner strongly suspects an allergy. Prick tests are generally not used when a reaction is considered life-threatening.

"False positives," where a food someone tolerates well returns a positive test result, do occur. An allergist will make a final diagnosis of an allergy based not just on the prick test, but also on the patient's symptoms and history.

How Prick Tests Work to Diagnose Allergies

Prick tests (sometimes called scratch tests) are performed on the skin of the forearm or the back. Allergists put a small amount of an extract of a potential allergen into a shallow scratch, using a plastic needle or sharp probe (it feels a bit like someone scratched you with their fingernail).

This type of testing is efficient: It's possible to test for multiple allergies at once since each one can be tested with its own scratch. In addition to food allergies, prick tests can look for allergies to pet dander, dust, mold, and pollen.

Within 20 to 30 minutes, a positive result will show as a hive, or wheal, on the scratch. The size of the hive may correlate with the intensity of the allergic reaction. Reactions may feel a bit like a mosquito bite (yes, they may itch).

In the event you have a severe reaction to the prick test, your allergist will administer a rescue medication such as epinephrine or an antihistamine. Severe reactions to a prick test, however, are uncommon. (This is why they are generally not used to confirm life-threatening reactions -- blood tests are much safer for that purpose.)

How Accurate Are Prick Tests?

Negative results are fairly accurate: if you don't react to the prick test of a suspect food or substance, it's unlikely you're allergic to it (although your physician may still follow up with more sensitive testing).

However, false positives on skin prick tests are much more common — they occur half the time or more, in fact. So a positive result on a skin prick test should always be coupled with careful analysis by your allergist to determine whether it's indeed a true result, or if it's a false positive.

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Article Sources
  • Food Allergy Research & Education. Skin Prick Tests fact sheet. Accessed Jan. 9, 2016.
  • U.S. National Library of Medicine. Allergy Testing fact sheet. Accessed Jan. 9, 2016.