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Blood Tests for Allergies

Our physicians at McGovern Allergy and Asthma Clinic may use a variety of testing techniques in order to best diagnose and treat a patient's allergies. While skin tests, including punch (prick) tests, are the most common way to pinpoint a patient's allergies, in certain cases a blood test, or RAST (radioallergosorbent) test may be the better option.

The first step in allergy testing, of course, is a discussion of medical history with your doctor; based on the results of this discussion, your allergist will determine whether further testing is necessary. With a skin test, tiny amounts of various allergens are introduced into a patient's skin, where a positive reaction (redness, swelling, itchiness) indicates the presence of an allergy. The results of this type of test are nearly immediate. Blood tests, by contrast, require only a single prick with a needle. The patient's blood is drawn and sent to a lab for analysis.

At the medical lab, the blood is tested for the presence of IgE antibodies, a type of antibody carried by allergic individuals in greater amounts. Possible allergens are placed on a paper disk and introduced into the blood sample. If the patient reacts to the foreign substance, IgE antibodies will attach themselves to the allergen. Once attached, radioactive IgE antibodies are added to the sample, binding themselves to the allergen-antibody clusters. After the paper disk is removed and washed, the level of radioactivity is measured by the analyst.

The RAST test is less widely used than skin testing because it is more expensive, slower to yield results, and generally less sensitive. Patients for whom a blood test remains the best option include those with severe skin conditions such as eczema or psoriasis, or those who for any other reason cannot tolerate skin testing. Certain medications can also inhibit accurate skin testing results, so if a patient is unable to skip the medication for a period of several days, a blood test may be given instead. To find out what type of allergy testing is appropriate for your situation, please contact our office to schedule an appointment. One of our allergy specialists will be glad to advise you on the best methods of diagnosis and treatment.