A RAST test is a type of blood test used to test for allergies. These tests, which use radioactivity, have largely been abandoned by doctors in favor of newer, more accurate forms of allergy testing, but some doctors still use them in certain cases.

RAST stands for “radioallergosorbent.” RAST tests have been a very safe way to test for food allergies, but they are also expensive and are not considered as accurate as food challenges.

When Are RAST Tests Used?

In the past, RAST tests were often used in combination with skin tests, or in situations when other tests are considered risky (e.g., when a patient has experienced a severe allergic reaction after eating a food). They’re still used occasionally in these circumstances, even though in the vast majority of cases doctors order ELISA tests instead of RAST tests.

Advantages and Disadvantages of RAST Tests

RAST tests are considered somewhat less sensitive than skin prick tests, although they are considered basically accurate and useful. They are valuable because they can give information about the level of allergic reaction present in a patient’s blood, and they don’t take much time.

RAST tests also can be used in some circumstances where prick tests cannot—for example, it’s possible to use a RAST test when the person being tested has severe eczema or persistent hives throughout the body that could throw the prick test off, leaving the prick test results in question.

Neither prick tests nor RAST tests, however, are considered as accurate as “blind” food challenges, where you don’t know what food you’re consuming. Where possible, a positive RAST test will be often be confirmed with a food challenge. In addition, RAST tests don’t exist for every potential allergen, so if an unusual allergy is suspected, your doctor may not be able to test for it using a RAST test.