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Test may reveal life-threatening peanut allergy

Scientists are edging a bit closer to developing a test that will reveal precisely who has a life-threatening peanut allergy, an issue that bedevils doctors and haunts parents.

New research, published in the latest edition of the Journal of Allergy and Clinical Immunology, suggests that testing for a molecular component of the peanut called Ara h 2 is an excellent predictor of true peanut allergy.

The research team, led by Adnan Custovic, a professor of allergy at the University of Manchester in Britain, said the test was able to identify 97 per cent of those at risk of anaphylaxis from exposure to peanuts.

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That is noteworthy because it is notoriously difficult to identify the children at real risk.

For example, in the new study, about one in 10 children had a positive skin or blood test that indicated a peanut allergy, but when they underwent an oral peanut challenge, only one in 50 had a true peanut allergy. (The balance tended to have allergies to grass and tree pollen.)

"The lack of specificity of current tests when used in isolation indicates many patients will inappropriately be given the diagnosis," Dr. Custovic said.

"The new diagnostic test, which accurately discriminates peanut allergy from tolerance, will mean we can target avoidance to those patients really at risk and remove the considerable stress that comes from the many false positive sensitivity tests."

The test used in the study is manufactured by Phadia AB, a company based in Uppsala, Sweden.

Susan Waserman, an allergist at McMaster University in Hamilton, said the new research is intriguing but the notion that an Ara h 2 test could definitely identify those at risk of an anaphylactic reaction to peanuts should be greeted with caution, particularly because the study involved only 19 children with severe peanut allergy.

"We don't know yet if this is the magic bullet," she said. "But the good news is that we're moving in the direction of much more specific diagnostic tests."

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Currently, testing for peanut allergy is quite crude. Trace amounts of peanut are rubbed on the skin and antibodies are measured in blood tests. Family history also offers important clues about the severity of risk.

Dr. Waserman said the paradox is that those at greatest risk of anaphylaxis are currently the easiest to identify because they have the most obvious readings in current skin and blood tests.

The real challenge is figuring out the risk of those whose readings are not at the top of the scales, and it is not clear if the new testing method makes that distinction.

Dr. Waserman added that even if the new test works well, it will not eliminate the need for an oral challenge, in which a person ingest peanuts in a clinical setting to determine if they have a severe reaction.

"That's still the only way to figure out who's truly allergic. I'm not sending my patients out into the world without it," she said.

The new research involved 933 eight-year-old children who have been enrolled in the Manchester Asthma and Allergy Study. To date, 110 of the children (11.8 per cent) have been identified as peanut-sensitized. But only 19 children (2 per cent) were found to have peanut allergy that placed them at risk of anaphylaxis.

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In Canada, an estimated 6 per cent of children and 4 per cent of adults suffer from food allergies, and the numbers are on the rise.

During an allergic reaction, certain proteins in the offending food cause the body's immune system to react by releasing chemicals such as histamine. Histamine can trigger swelling, hives, eczema, nasal congestion, wheezing, asthma, nausea and vomiting. Symptoms usually develop within a few minutes to one hour after eating the food.

Anaphylaxis is a severe allergic reaction that occurs rapidly, can involve only a trace amount of food and affects the whole body. Without immediate attention, death can result.

Children who are sensitive to milk or eggs tend to lose their allergy over time. Allergy to peanut, tree nuts, fish or seafood tends to persist.

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