Radioallergosorbent Test

The radioallergosorbent test (RAST) measures immunoglobulin E (IgE) antibodies in serum by radioimmunoassay and identifies specific allergens that cause rash, asthma, hay fever, drug reactions, and other atopic complaints. RAST is easier to perform and more specific than skin testing; it's also less painful for and less dangerous to the patient. Careful selection of specific allergens, based on the patient's clinical history, is crucial for effective testing.

Although skin testing is still the preferred means of diagnosing IgE-mediated hypersensitivities, RAST may be more useful when a skin disorder makes accurate reading of skin tests difficult, when a patient requires continual antihistamine therapy, or when skin tests are negative but the patient's clinical history supports IgE-mediated hypersensitivity.

Purpose

• To identify allergens to which the patient has an immediate (IgE-mediated) hypersensitivity
• To monitor the patient's response to therapy

Patient Preparation

• Explain to the patient that this test may detect the cause of allergy or monitor the effectiveness of allergy treatment.
• Inform him that he needn't restrict food or fluids.
• Tell him that the test requires a blood sample and who will perform the venipuncture and when.
• Reassure him that although he may experience transient discomfort from the needle puncture and the tourniquet, collecting the sample takes less than 3 minutes.
• If the patient is scheduled for a radioactive scan, make sure the sample is collected before the scan.

Procedure And Posttest Care

• Perform a venipuncture, and collect the sample in a 7-ml red-top tube. Generally, 1 ml of serum is sufficient for five allergen assays.
• Note on the laboratory slip the specific allergens to be tested.
• If a hematoma develops at the venipuncture site, apply warm soaks

Reference values

RAST results are interpreted in relation to a control or reference serum that differs among laboratories.

Abnormal Findings

Elevated serum IgE levels suggest hypersensitivity to the specific allergen or allergens used.

Interfering Factors

• Radioactive scan within I week before sample collection