About Allergy Blood Test | Radioallergosorbent Test

BLOOD TESTS

Complete Blood Count (CBC)

This basic test counts red and white blood cells, and checks for many diseases and ailments. A high number of eosinophils (a type of white blood cell) suggests allergy.

Radioallergosorbent Test (RAST)

Insurers haven't always been willing to pay for the RAST (radioallergosorbent test), which may or may not cost more than skin testing, depending on the number of substances tested and the doctor's markup of lab charges. This once-controversial di­agnostic tool has graduated to general acceptance, however, with FDA approval and even a cautious nod from the conserva­tive JAMA.

Like skin tests, RASTs have benefits and limitations. Accord­ing to Dr. Edward O'Connell, professor of pediatrics, allergy, and immunology at the Mayo School of Medicine, "RAST is practical, easy on the patient, and effective even if a patient has a body rash or other symptoms, or is taking antihistamines. Some of the newer medications can affect skin-test results for up to two months."

Experts agree that while skin tests sometimes give false positives, especially to foods, RAST can give false negatives— that is, it may miss substances you are allergic to.

RAST has many spin-offs, such as ELISA (enzyme-linked immuno-assay test), known for its ability to detect the human immunodeficiency virus (HIV); and PRIST (paper radioim­munosorbent test), which measures the total number of IgE an­tibodies in the blood.

Dr. Vincent A. Marinkovich, diplomate, American Board of Allergy and Clinical Immunology, has helped develop a version of RAST called MAST—multiple antigen simultaneous testing.

"MAST is more comprehensive and sensitive than RAST," he explains. "RAST deals with single allergens and MAST can test thirty-eight at a time. It's not diagnostic for foods, but it can direct you to areas where there may be problems."

Having been skin-tested all my life, and being quite familiar with my allergies, I decided to try MAST and check its accuracy. Dr. Marinkovich agreed to take a blood sample, and knowing nothing about me or my medical history, he ran it through the lab.

The results were impressive; I'd say 8.5 on a scale of 10. MAST picked up my allergies to cats, dust mites, molds, and pollens, but faltered slightly on foods, correctly pinpointing corn and eggs but missing wheat. No skin test or in vitro test to date, however, claims to be wholly accurate for foods.

"The ideal procedure," says Clyde Thorn, a nuclear engineer and biochemist who does testing for five Northern California al­lergists, "is to use the prick test or RAST as a screen in connec­tion with the medical history to give some idea where to start. Then I follow up with intradermals using serial endpoint titra­tion (SET)."

Also called serial dilution, SET involves giving the patient various strengths of the allergen until there's little or no reac­tion. That strength is then used as the starting dose for im­munotherapy.

As to which test is superior, Dr. O'Connell states, "Skin tests and RAST are very comparable. Both are 90 percent reliable and effective as diagnostic tools."

Nevertheless, the American Academy of Allergy, Asthma and Immunology (AAAAI) warns that both skin and RAST test­ing are open to abuse most commonly, unwarranted im­munotherapy for persons with weakly positive test results and no evidence of allergic disease.