What Is a RAST Test?

The RAST allergy test, or radioallergosorbent test, measures the level of allergen-specific IgE antibodies in your blood. Simply put, a RAST test measures your immune system's reaction to a particular food allergen.

Many healthcare providers use skin tests to screen for allergies. That's because RAST and another blood allergy test called ImmunoCAP are less reliable screening tests with many false positive results. However, a healthcare provider may have a reason to order an IgE blood test—for example, to test 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.

How Is RAST Performed?

One of the central roles of the immune system is to produce proteins called antibodies to fight any substance or organism it views as a threat. When you have an allergy, your immune system will mistakenly regard a harmless substance (allergen) as a threat and produce immunoglobulin E (IgE) antibodies specific to that allergen.

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.

RAST Tests vs. Skin Tests

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

Skin testing is the most often used by allergists 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.

Keep in mind that not all tests are used for the same types of allergic reactions. For example, the skin prick test typically is used to diagnose allergies to food, medication, or environmental irritants such as pollen.

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

Your healthcare provider will also help you understand the pros and cons of each test. For instance, skin prick tests tend to be less expensive than blood tests. (Your costs will depend on your insurance coverage.) However, they also test for fewer allergens than blood tests.

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.

RAST Testing Considerations

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.

A Word From Verywell

RAST tests are just one way of testing for food allergies. They 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.