Are blood tests for food allergies a waste of time?

This is a question that came up last week when talking to a friend with some digestive issues surrounding specific foods. It's a bit of a complex topic, so I'll try to boil it down to the basics.

First of all, the term "food allergy" can manifest in different ways. It's like saying "vehicles". There are cars, which are common and generally well understood, but there's also helicopters, airplanes, and hovercrafts that we don't see everyday and most people don't have a clue how they work.

The first category of food allergies (lets call them the cars) are the classic ones that are mediated by histamine. They develop rather quickly and produce symptoms in the skin (hives, swelling), the gastrointestinal tract (nausea, vomiting, diarrhea, itchy mouth), and in the cardiopulmonary system (anaphylaxis). Peanut allergies are a common example. Respiratory allergies such as hay fever are mediated by the same mechanism.

The other category (helicopters and hovercrafts) is mediated by another class of antibodies called IgG. I like to think of these as personalized "kick-me" signs that your body makes for each foreign invader it has seen before. Of all the antibody classes, these are the most abundant in the body and their job is to activate the immune system to destroy the invader quicker than the first time you were exposed. So how are these related to food?

Although the human gastrointestinal tract was once thought of an impenetrable brick wall, the reality is that it can become damaged, and partially digested food comes in contact with the immune system. This can result in the body making the "kick me" signs against food proteins. Upon subsequent encounters, these food particle/"kick me sign" complexes clump together and circulate around and eventually deposit themselves somewhere, producing a myriad of symptoms that can seem totally unrelated to your GI system. These include skin rashes, fatigue, joint pains, and many others. This form of allergy is less well understood and the symptoms can be less predictable. It is also not yet an exact science.

As for the food allergy testing, the first type is done by most allergists by skin-prick testing. The second type (IgG) is done from the blood, usually ordered through specialty labs. The bottom line is that the tests have yet to be perfected and a lot hinges on the specific lab's methodology. No test is perfect, and this is certainly no exception. That said, they can provide a doctor with some useful information. For example, perhaps someone tends to show reactivity to oranges and grapefruits, suggesting that the whole citrus family may be problematic for them. In another case, someone may show reactivity to almost all foods on the panel. This would indicate that their digestive tract has become leaky (for lack of a better word) at some point, and they need to start by healing it. If nothing else, they provide a starting point for doing a clinical food elimination and re-challenge, which is after all the most certain way of identifying problematic foods. A recent review was published out of John's Hopkins University School of Medicine came to a similar conclusion - though diagnosing food reactions remains a challenge, IgG food allergy panels have shown promise for directing elimination diets, with clinically meaningful results.

Long as this post has become, it is far from comprehensive. If you suspect you have difficulty with some foods, I recommend seeing a medical professional with expertise in this area.

Regards,

Bryn