Allergists are hopeful, but cautious, after a new Toronto-led study suggests we may soon have a test to predict who among the allergic is at most risk for a serious reaction, known as anaphylaxis.
Published in the New England Journal of Medicine in January, the study showed levels in the blood of a chemical called platelet-activating factor (PAF) along with levels of the enzyme that destroys PAF, have a relationship with the seriousness of symptoms during a reaction.
The study’s lead author, Dr. Peter Vadas, director of allergy and clinical immunology at St. Michael’s Hospital, explains that anaphylaxis is a “cast of many characters.” There are at least two lead actors: PAF is a “bad guy,” who brings on life-threatening symptoms like dropping blood pressure and a swelling airway, while PAF acetylhydrolase (PAF-AH) is the “good guy,” who stops PAF from setting off mayhem.
High levels of PAF had already been found in animals experiencing anaphylaxis, and scientists learned they could bring about symptoms of anaphylactic reaction by giving PAF to healthy animals. As well, drugs to block PAF have prevented allergic reactions in animals. But Vadas’s latest study, which was eight years in the making, is the first to demonstrate similar responses in humans. “With confirmation of these results, I think we’re going to have a diagnostic test that will help us to stratify the risk of having life-threatening or severe anaphylaxis,” he says.
The researchers on his team measured the blood levels of PAF and the good enzyme PAF-AH in 41 patients who had arrived in the emergency room having an anaphylactic reaction and compared them with blood taken from 23 healthy volunteers. They found that PAF levels went up with increasing severity of anaphylaxis, and PAF-AH levels went down with increasing levels of severity of anaphylaxis. Vadas describes it as a “yin yang” relationship.
Researchers also tested blood samples from 215 previous patients, including nine people who died from anaphylaxis after eating peanuts. The nine who died had significantly lower levels of the good enzyme. From a group of 63 children who’d had only mild peanut reactions, PAF-AH levels were found to be almost as high as those in people who had no allergies at all.
Once an independent group of researchers confirms the finding, Vadas says that within a year, allergists should be able to offer a blood test for PAF-AH levels. The results would provide specialists the first ever indicator of whether the person has this specific biochemical marker for the risk of a life-threatening reaction. Vadas compares it to being able to make similar assessments in other medical conditions – for instance testing for cholesterol, a specific marker of heart disease. Doctors would consider PAF-AH test results in context with other risk factors, such as a history of reactions and asthma, and give allergic patients a better prediction of how likely they are to be in jeopardy when a bee stings or if they accidentally drink milk.
Vadas is quick to point out that PAF and the enzyme that stops it are just two components ….
Excerpted from the Spring 2008 issue of Allergic Living magazine.
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