I'd imagine this happens all the time. I don't blame patients at all. Who cares most about your health? You do. Do you think your medical providers, whatever system you use, are doing as well as they could for you? Probably not. So if you've got a chronic condition that seems to be related to what food you eat, you get a blood test done to try to find the culprit(s).
There's more than one problem with this, however. Most patients don't understand the difference between food allergy, reaction, and sensitivity. The primer's author, Elana Lavine, briefly explains:
Food allergy is an adverse health effect arising from a specific immune response that occurs reproducibly on exposure to a given food. Nonimmunologic adverse reactions to food are termed food intolerance and include conditions such as lactase deficiency, dietary protein–induced enterocolitis syndromes and eosinophilic gastrointestinal disease. Food sensitivity is a nonspecific term that can include any symptom perceived to be related to food and thus may be subject to a wide range of usage and interpretation.Eczema patients (in my understanding), are usually subject to the first and third categories. We probably have IgE antibody-based allergies to one or more foods (in addition to pollen and pet dander). And our skin inflammation is exacerbated by anything that causes increased blood flow at the surface, such as spices, alcohol, and histamines from fermented or aged foods such as Parmesan.
Lavine says that many patients get blood tests based on measuring levels of IgG --a separate class of antibodies from IgE. But, she says,
neither total IgG nor IgG4 [a subclass] levels correlate with food allergy as shown on double-blind placebo controlled food challenges.IgG4 floating around in our systems may just mean that we have been exposed to a food and become able to tolerate it.
I did a quick web search and found a company called ALCAT that claims to test for food sensitivity using their proprietary technology, which measures how white blood cells change size when exposed to antigen. I'd have to consult an expert but I have trouble believing that results from such a test would truly reflect how your body is reacting--or not--to foods.
So what can you do to determine whether food is causing your eczema to flare up? Lavine says
Making the diagnosis of a specific food allergy may include the following: a full medical history, physical examination, skin prick testing, carefully selected food-specific IgE levels and oral food challenges to suspected food allergens in some instances.There's no easy answer. To nail an allergy, you need the whole shebang. Avoiding a food entirely and seeing whether your eczema improves is a good start. The trick is, in my experience, to eat as few processed foods as possible so you can get control of the ingredients. Beware: even something as simple as soy sauce may contain wheat, for example.
[added later] I realized that I didn't address the topic of IgE tests. I don't think that a consumer (in the US) can get this test done without a referral from a doctor. The last time I asked an allergist about getting tested for IgE (the technology available at the time was RAST), he told me that "they" didn't do RAST on atopic patients because the circulating level of IgE was so high that it inevitably saturated the measurement no matter what they tested for. However, a quick Google search reveals that Quest Diagnostics now touts the ImmunoCAP blood test as a way to determine IgE allergy. Has anyone tried ImmunoCAP?