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Tuesday, February 03, 2009

Blood Tests Overdiagnose Allergies

Tara Parker-Pope's [corrected name] column in today's New York Times makes a point that every parent of an allergic child - or possibly allergic child - should read.

The blood tests that many parents are turning to as an easier and faster diagnostic for food allergies than the old skin prick tests aren't reliable. They identify many children as being allergic when they really aren't. These are known as false positives.

A 2007 issue of The Annals of Asthma, Allergy & Immunology reported on research at Johns Hopkins Children’s Center, finding that blood allergy tests could both under- and overestimate the body’s immune response. A 2003 report in Pediatrics said a positive result on a blood allergy test correlated with a real-world food allergy in fewer than half the cases.

“The only true test of whether you’re allergic to a food or not is whether you can eat it and not react to it,” said Dr. David Fleischer, an assistant professor of pediatrics at National Jewish Health. ...

Blood tests may be unreliable because they fail to distinguish between similar proteins in different foods. A child who is allergic to peanuts, for instance, might test positive for allergies to soy, green beans, peas and kidney beans. Children with milk allergies may test positive for beef allergy.

Nobody is saying that the blood test shouldn't be used. Instead, it should be used as a first method of screening for allergies. A quick, easy test that shows up negative means that your child is spared longer or more invasive tests.

However, further testing is necessary if a number of allergies are detected. Usually this testing involves food challenges. Foods that have been removed from the child's diet because of a positive result are put back into the diet, carefully and in small quantities, to see if a reaction occurs. Reactions to actual food are the best evidence of an actual allergy. Remember that very young allergic children often outgrow allergies to milk as well as to eggs, soy and wheat.

Reintroducing food should only be done under the supervision of a doctor. If the child remains allergic a severe reaction could occur and doctors can control this.

The risks of a serious allergic reaction are real. However, so are the risks of taking children off of common foods unnecessarily. Cases of malnutrition have been reported from overly restricted diets. And the chances of a reaction when the food is reintroduced - deliberately or accidentally - are increased.

Allergies are increasingly widespread. That also means that some parents are assuming allergies where they don't exist, creating a vicious cycle. Doublecheck all positive results from testing, especially from blood tests. And make sure they are real tests from the best trained doctors. There are too many quacks preying on peoples' fears to take any chances with your child's health.

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