RAST (Radioallergosorbent Test; Serum Specific IgE)

• Introduction to RAST (serum specific IgE)
• When is RAST used?
• Preparing for a RAST
• How is a RAST performed?
• What do the results of a RAST mean?
• Risks of a RAST

Introduction to RAST (serum specific IgE)

Avoiding allergens is an important part of the management of allergic diseases such as food allergy, asthma and allergic rhinitis (hayfever). It is therefore important to know what you are allergic to.

Radioallergosorbent testing (RAST) detects allergen specific IgE in the blood. They are used when skin prick tests (the preferred allergy test) is not suitable or not available. It is important to remember that allergy tests cannot be used on their own and must be interpreted in conjunction with symptoms and clinical history. Medicare rebates are available for RAST.  

Important note: Unproven allergy tests

There are several methods that claim to test for allergy including cytotoxic food testing, kinesiology, Vega testing, electrodermal testing, pulse testing, reflexology and hair analysis. These tests have not been scientifically validated and may lead you to take unnecessary, costly and (in the case of some changes in diet) dangerous avoidance strategies. No Medicare rebate is available in Australia for these tests and the use of these methods is not advised.

When is RAST used?

RAST is used to confirm what a patient may be allergic to, which can improve treatment of allergic diseases such as asthma and allergic rhinitis, when skin prick tests (the preferred allergy test) is not available or is not suitable. This includes cases where facilities are not available to manage severe reactions to skin-prick testing, for people with certain skin conditions such as severe eczema, or when a person is taking medications (such as antihistamines) that interfere with accurate testing and it is not suitable for them to stop taking their medication temporarily.

Preparing for a RAST

Before skin prick testing is performed, your doctor will take a careful and detailed clinical history looking for relationships between exposure to various allergens and allergic reactions.

The actual test itself is a simple blood test and no other preparation is required.

How is a RAST performed?

RAST is performed on blood taken as a simple blood test. Blood is drawn from a vein (venepuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to fill with blood. A sterile needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding. Some people will experience bruising around the site.

Results are generally not available for a few days to a week and it is necessary to make another appointment with your doctor to discuss the results.

What do the results of a RAST mean?

RAST testing is done against a group of allergens, such as a food allergy screen, or against a particular allergen, such as house dust mite. If the test is negative, it is likely that you do not have the ability to mount an allergic reaction to that allergen and any symptoms you are experiencing are probably not due to an allergic reaction to that substance.

If the test is positive for a specific allergen, such as house dust mite, it shows that you are sensitised to that particular allergen. This means that you have the ability to mount an allergic response when you are exposed to that allergen. It does not mean that you will necessarily have a clinical allergic reaction to that substance, however, nor does it mean that any reactions will be severe.

The results of the RAST need to be interpreted in conjunction with your clinical history and pattern of allergic reactions.

Risks of a RAST

The risks associated with RAST are limited to the risks of having a blood test done, and include:

• Excessive bleeding (more likely if you have a bleeding or clotting disorder)
• Fainting or feeling light-headed (this usually resolves quickly)
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins (venepuncture is easier on some people than others)

References

1. ASCIA Education Resources. Patient Information Brochure: Allergy Testing. 2003. Australasian Society for Clinical Immunology and Allergy. Available at: http://www.allergy.org.au/aer/infobulletins/allergy_testing.htm
2. ASCIA Education Resources. Health Professional Information Brochure: Laboratory Tests. 2005. Available at: http://www.allergy.org.au/aer/infobulletins/Laboratory_Tests.htm