How a RAST Test Diagnoses Allergies

A RAST test (radioallergosorbent test) is a blood test used to check for allergies. It measures your immune system's reaction to certain exposures (such as food allergens) and the responses that lead to allergy symptoms.

Many healthcare providers use skin tests to screen for these immunoglobulin E (IgE) antibody responses in your blood. That's because RAST and other blood allergy tests (like ImmunoCAP) are less reliable screening tests. However, RAST can be an alternative to a skin prick in some cases, as with children who don't tolerate skin tests well.

This article explains how the RAST test is performed, along with its limitations. It also discusses how to read a RAST test and what the values on your test may indicate.

What Is a RAST Test?

RAST stands for radioallergosorbent test. It may be used instead of skin tests to detect a food allergy in specific groups, including people with severe eczema and those who need to stay on antihistamine medications.

RAST also can be used to monitor IgE levels in people who already have a food allergy diagnosis.

Besides RAST, other blood tests for allergies include the enzyme-linked immunosorbent assay (ELISA) or fluorescent enzyme immunoassay (FEIA) tests. The ImmunoCAP test is a preferred method, allowing for molecular-level diagnosis in support of more targeted treatment options.

RAST Tests vs. Skin Tests

A number of skin tests may be used to diagnose an allergy, rather than relying on the RAST test.

Allergy Skin Tests

Skin testing is the most often used by allergists (allergy specialists) because it's considered the most accurate. However, there are reasons that blood tests may be ordered instead. For example, young children may be less likely to cooperate with a skin prick test.

Skin tests include:

• Skin prick testing, with allergen exposure through light skin scratches of the back or arm
• Intradermal testing, with an allergen injected into a deeper skin layer to check for reaction
• Patch testing, with a bandage containing allergen applied to the skin for testing

If a skin test is positive, you'll find a red, raised area where reaction to the allergen occurred.

Your healthcare provider will choose a test based on the suspicion of a certain type of food or other allergy. That decision will be based, in part, on your overall health history, your symptoms, and the findings of their physical examination.

Patch testing is helpful when diagnosing allergies caused by skin contact, certain drugs, or even implantable devices used in knee replacement surgeries.

RAST Tests for Allergies

RAST tests are used for many of the same reasons that skin prick tests are. The allergy blood tests in general are considered to be comparably effective as the skin pricks are for food and airborne (respiratory) allergies. But they should be used alongside of the intradermal test when evaluating an allergy to medication or an insect venom (like bee stings).

RAST and other blood tests may be better than skin tests if:

• You have an unstable or poorly controlled underlying medical condition, including skin problems
• You have severe allergies with a high risk of anaphylaxis, a potentially life-threatening allergic response.

People who take certain medications may need to have a blood test rather than a skin test. Xolair (omalizumab), often used to treat urticaria (hives) and newly approved by the Food and Drug Administration for food allergies, can lead to false positive skin prick tests for up to a year when prescribed. The long-term use of steroids can interfere with skin tests.

What Can RAST Test For?

Blood tests like the RAST test can be used to test for a number of allergen types, including:

• Pollen, dust, and mold in the environment
• Animal dander (typically pet hair)
• Certain foods known to cause allergies, like peanuts and shellfish
• Certain drugs and medications

The RAST test also can help when diagnosing people with asthma symptoms, due to the strong link between allergies and asthma in both children and adults. Tests may be geared toward irritants that you inhale, like dust or mold, or to food allergies. Your healthcare provider can discuss the most appropriate test if you have asthma.

How Is RAST Performed?

When your healthcare provider orders a RAST test, you will be asked to provide a blood sample. The lab will then use the RAST technology to look for allergen-specific IgE antibodies. This is done by adding specific allergens to the blood sample to see if IgE is produced in response.

Then, the IgE levels in your blood sample are measured. These values are recorded in the RAST test list on your lab report.

How Accurate Is the RAST Allergy Test?

As straightforward as a RAST test may seem, it has some definite limitations. While a RAST test can detect the concentration of IgE antibodies in your blood, it cannot predict how you will respond to the allergen associated with it.

Not everyone with the same concentration of antibodies will react in the same way, either. In some cases, a person with a relatively low concentration may have a severe reaction when exposed to an allergen.

Or, someone with a high concentration may react mildly or not at all. In this case, the person isn't truly allergic even if the blood test is positive. People with low concentrations of allergen-specific antibodies tend to have a low chance of reacting to a possible allergen in daily life, especially if a skin prick test is also negative.

Interpreting Your Results

Different foods have specific IgE levels that are considered "predictive" in the RAST test. This can be determined by comparing IgE concentrations to a reactive allergic response in a food challenge.

Doing so can determine at which lgE level a person is more likely to experience an allergy to a specific allergen.

These values provide labs with the numeric references they need to interpret a blood test result. Each allergen has its own reference value which the lab will interpret individually.

Disadvantages of RAST Test

A RAST test can be used to direct some aspects of allergy treatment. For example, they can sometimes help to determine whether a child is showing signs of outgrowing a food allergy. A RAST should never be used in isolation but rather to support the findings of other tests.

Although predictive values have been established for some foods, those levels sometimes vary by age. Moreover, researchers haven't determined the predictive values for all foods.

In the era of genetic medicine, other blood tests may be more accurate than the RAST in identifying immunoglobulin E (IgE) antibodies in a targeted way. Tests have become more sensitive and specific, leading to more personalized treatment options. These continue to evolve; for example, some tests have found food allergy-related IgE antibodies in human tears when they weren't detectable in the blood sample.

Summary

The RAST test is a blood test that can be used to check for allergies. Newer tests that have been developed, including the ELISA, have made the RAST test somewhat dated. It also can be less accurate than a skin test, though it may be a better option in specific circumstances, like allergies in children or people who take certain medications.

RAST tests may offer clues about your symptoms, but your allergist is likely to use them in combination with other allergy tests. If you have specific questions about your RAST test, your allergist or immunologist is the best person to ask.