Allergy tests for eczema a complex, controversial topic
People with eczema—or parents of children with eczema—often suspect that food allergies are somehow setting off eczema flares. But nailing down exactly which foods cause trouble is made difficult by the many other factors that can trigger eczema. Such triggers include emotional or physical stress, environment, and viral or bacterial infection. Recently I spoke on the phone to Jon Hanifin, a professor of dermatology and clinician at Oregon Health Sciences University and NEA board member, to find out whether allergy tests can truly benefit eczema patients.
The upshot is that there is no easy answer, although it is possible to use a standard skin prick test or serum RAST assay as part of the process to diagnose true food allergy. Caution is essential. False positives, for example, can cause parents to needlessly avoid food that provides vital nutrients for growing children, and to focus on chasing allergies rather than providing good skin care.
Hanifin said that many caregivers who do allergy testing face a conflict of interest. Are they prescribing large panels of allergy testing, or recommending allergy shots, because they would benefit the patient, or because the tests cost a lot of money? “I had an allergist talk to our residents a number of years ago,” Hanifin said, “and he said ‘an AD patient comes in to see me, they don’t get out for less than $400.’”
For doctors, Hanifin strongly recommends referring to guidelines published by the National Institute of Allergy and Infectious Diseases. “When you have a patient not responding to topical treatment, you should consider [allergy tests],” he said. “Or when you have a patient that seems to have had a reaction to a food—you need to confirm that.” I’ve skimmed the patient guidelines and they seem clear and useful.
The guidelines define a food allergy as “an adverse health effect arising from a specific immune response that occurs reproducibly on exposure to a given food.” A true allergy, as opposed to an “intolerance,” manifests within ten to 120 minutes, Hanifin says. Skin prick tests in the office are a relatively reliable—although far from perfect—indicator.
In a skin prick test, the nurse pokes your skin with an array of little needles loaded with extracts from various foods or allergy agents (e.g. rye grass). These extracts rarely consist of a single purified protein. Quite the opposite: “fish”, for example, is often a mix of extracts from cod, salmon, shrimp, etc. After the pricking, you wait in the doctor’s office for about 20 minutes, and then the allergist looks at the sizes of the various itchy red spots. “If there’s a whopping big reaction that’s going to influence them to say there’s an allergy; whereas if it’s a minuscule thing, it doesn’t mean anything,” Hanifin says.
Hanifin also says RASTs are sometimes a useful option. A RAST measures the amount of allergen-specific IgE antibody in your blood. “But it’s very imprecise,” he says. “Experts try to make it seem that if you have a certain level of antibody, you’re more likely to be allergic, and that hasn’t held up. People and companies develop these tests and market them commercially— that’s another possible source of conflict of interest.”
He added that he recommends doing only very small panels of tests. “If a child has had a reaction to one or two or three things, limit the testing to those,” he says. “It’s awfully tempting as you’re setting up to say ‘oh, let’s do all 50 of those,’ and naturopaths sometimes have over 100. The more you do, the more likely you are to get false positives and confuse everything.”
The best test of a food allergy is an oral challenge, done in the doctor’s office (with an EpiPen at hand). This is because a true allergic reaction is definitive and happens quickly. But, Hanifin says, “most of the time allergists are unwilling to do a challenge in the office; yet new studies have shown more than 80% of kids taken off foods because of allergy tests, are based on false positives. When you do a negative challenge in the office, it’s clearly not an allergy.” He is unsure why allergists are reluctant to do in-office challenges, and suggests that there may be little insurance reimbursement incentive.
Hanifin did recommend that I speak to an allergist or two to get their perspective, but that will have to come in a future post.
I have eczema myself, and I would like to make a point that I feel is not addressed by the NIAID guidelines. I know from experience that certain foods trigger my eczema. These include concentrated tomato; concentrated citrus; aged cheese; hot pepper; and alcohol of all kinds, but especially red wine. These reactions of mine are likely not allergies; nor are they intolerances like many people have to lactose in milk.
[Hanifin says of concentrated citrus and tomato: “these are obvious irritants—seldom allergens.” Of hot pepper “also an irritant, and causes autonomic sweat reactions.” Evidently I need to explore these types of food reactions further. ]
This is what I think is happening. In an allergic reaction, an allergen binds to IgE antibody; the antibody binds to receptors on mast cells, which release histamine, which dilates blood vessels, which causes inflammation and often hives. Foods (or other substances) can also bypass antibodies and set off the later events in this sequence. Many foods, especially aged or fermented ones, contain histamine. And alcohol and hot pepper can dilate blood vessels. The end effect is mostly the same: you get itchy.
I don’t care whether my reaction is classed as an allergy, an intolerance, or whatever. I just don’t want to have it.
The best way not to have a reaction is to identify the trigger and avoid it. Here, Hanifin has some advice for allergy testing that holds true for any kind of reaction.
“If you’re worried about a food and avoiding it,” he says, “then we try to get the skin clear”—by intensive moisturizing and topical steroids. This takes a few weeks. Then, once the skin has calmed, “if you didn’t have a clearcut, dangerous reaction, you can try taking that food again. And most likely it will be fine.”