What Is a RAST Test?

An Older Type of Allergy 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 blood tests, such as RAST or ImmunoCAP, are less reliable screening tests with many false positive results. However, a healthcare provider may have 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.

Blood tests in a tray
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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.

What Does RAST Stand for in Allergy Testing?

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

How Accurate Is the RAST Allergy Test?

As straightforward as a RAST test may seem, it has its 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 and Skin Tests

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

Keep in mind that 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.

In 2010, the National Institute of Allergy and Infectious Diseases (NIAID) recommended that more sensitive tests be preferred instead of RAST when diagnosing allergies. Further, the ImmunoCAP test can be used to prove someone is sensitized to an antibody, but not to prove an allergy exists in the absence of other confirmation.

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.

What Is a Positive RAST Test?

Most labs will report the RAST findings on a scale of 0 to 5 or more. A 0 value suggests a low likelihood of an allergy. Positive values mean there's a likelihood of an allergy, which increases with a higher number. Some labs report the IgE values in micrograms per milliliter (μg/mL). An experienced allergist will know what this means.

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.

Frequently Asked Questions

  • Are at-home tests used to diagnose allergies?

    At-home tests are available, and may provide information about sensitivity and food intolerance, but they cannot be used to diagnose a food allergy. At-home kits to test DNA for allergies also exist but their accuracy is not confirmed by research.

  • Can a RAST test be used for people with asthma?

    The Centers for Disease Control and Prevention recommends allergy testing for adults and children who have persistent asthma symptoms. 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.

  • Is the RAST test done for dogs, too?

    Yes, there are canine RAST tests for your pet.They exist for horses, cats, and other animals treated by veterinarians. Speak to your pet's vet if they show signs of allergy and you are interested in testing.

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9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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