This is an issue that most allergists run into on a fairly frequent basis- a parent walks into the office with bloodwork ordered by another physician. Tests returned as "positive" for multiple foods, so for the past 3 months, parents have restricted all these foods (generally there's always something that's a pain in the butt to eliminate, like soy or wheat). Now we have a child who is still as symptomatic as ever, and cranky to boot.
When I tell the parents that their child in all likelihood is not allergic to all of the eliminated foods, the reactions are a mixture of relief and annoyance: "That's great news, but why did we just spend 3 months of our lives wasting a good portion of our paychecks in the gluten/wheat-free aisle at Whole Foods?"
The answer: Because the blood test was ordered without a full understanding of how to interpret the results. The presence of IgE to a specific food does not always translate into clinical allergy.
It is possible to become sensitized to a food, but tolerate it without issue. In these cases, it can actually be counter-productive to eliminate this food, because continuing to eat it in small amounts may be maintaining a state of immune tolerance.
There are multiple nuances in the diagnosis and management of food allergy that simply cannot be delineated by a blood test.
So, why are these blood tests ordered so frequently? The reasons are numerous:
- Physicians want to help their patients by finding the source of a problem. We're detectives by nature. Because the vast majority of primary care physicians do not have the capability to offer skin testing (more accurate than blood testing) in their offices, blood testing seemingly offers a simple way to provide the same service to their patients.
- There is a misperception among both physicians and the lay public that allergy skin testing is a painful, traumatic process. Physicians and parents feel that they are sparing the child an invasive procedure by choosing a blood test instead. This is inaccurate- allergy skin testing is needleless and bloodless. In the case of a pediatric panel, the testing takes only seconds to apply, and 20 minutes to get results. In contrast, a blood draw requires temporarily restraining the young child while the phlebotomist uses a needle to access a vein (more painful than the superficial scratch from the plastic skin testing device), and then waiting for 1-2 weeks until the test results are delivered to the ordering physician.
- There is the inaccurate assumption that blood testing is more economical that skin testing. This is certainly wrong. The average cost for an individual Phadia Immunocap blood test is $100. In contrast, the average allergy skin test is $10 per item. Some argue that the cost of the blood-based food panels are less than ordering the same tests individually- however, there is generally not value in ordering a panel test which includes foods that the child obviously tolerates- it is a complete waste of resources. In my office, I do not skin test a child to a food that they tolerate without issue. Even when you factor in the cost of the allergist's office visit, skin testing offers a better value, with less wastage of health care resources.
The scenarios in which I use blood allergy testing:
- A child has such extensive eczema that there is not sufficient clear skin on which to apply the test.
- The history of reaction was so immediate and severe (example- life-threatening anaphylaxis from minimal peanut exposure) that it is not prudent to risk the small risk of a systemic reaction from the skin test if the allergy can be confirmed by blood test instead.
- For whatever reason, a child is not able to discontinue antihistamines prior to skin testing.
