Allergy Testing for Babies: Skin Prick and IgE Blood Tests

Both tests hunt for IgE, the antibody behind fast allergic reactions such as hives or swelling soon after a food. For a skin prick test, Food Allergy Research & Education (FARE) describes the allergist placing a drop of food extract on the forearm or back, then using a small plastic probe or needle to prick the skin so a tiny amount gets just below the surface. Results usually appear within 15 to 20 minutes. A positive is a wheal, a raised white bump with a small ring of itchy red skin around it.

The blood test measures specific IgE antibodies to each food in a lab. FARE notes that antihistamines can interfere with skin testing, and that where stopping a medicine for a while would make symptoms worse, the doctor may choose a blood test instead. The NIAID-sponsored food allergy guidelines recommend both tests to help identify foods that may be provoking IgE-mediated reactions. The same guidelines attach a condition to each one, and that condition is where most of the confusion starts.

The condition is short. NIAID says the skin prick test "alone cannot be considered diagnostic" of food allergy, and of specific IgE blood tests it says "alone these tests are not diagnostic of FA." The guidelines explain why in their definitions: a person can carry IgE to a food and eat it without any symptoms. "Sensitization alone is not sufficient to define FA." An allergy needs both the antibody and a reaction when the food is eaten.

FARE puts a number on the gap: about 50-60 percent of all skin prick tests give a false positive, showing positive for a food the person is not allergic to. A bigger wheal leans more towards true allergy, FARE adds, but its size is not always an accurate predictor.

So a result is read against what happened at the table. A positive for egg in a baby who broke out in hives after scrambled egg means something quite different from a positive in a baby who eats egg muffins every week. When the two stories do not line up, a supervised oral food challenge is how allergists settle it. NIAID calls the double-blind version the gold standard.

It sounds like the thorough option, and it is the one the allergy bodies warn against. In its Choosing Wisely list from 2012, the American Academy of Allergy, Asthma & Immunology (AAAAI) wrote: "Don't perform unproven diagnostic tests such as immunoglobulin G (IgG) testing or an indiscriminate battery of immunoglobulin E (IgE) tests in the evaluation of allergy." Its reasoning: "Skin and blood testing to foods has a high rate of showing positive results in people who don't have food allergy." Allergy diagnosis instead "requires specific IgE (skin or blood) testing guided by the patient's history and physical examination."

The point about false positives is the same one FARE makes about skin tests in general. Put forty foods on one form and every stray positive lands on a food, sometimes one that is already going down happily at dinner. NIAID adds a second reason to be careful with assumptions, noting that multiple studies show "50% to 90% of presumed FAs are not allergies." For us, that figure made a broad panel look less like reassurance and more like a list of new worries.

The kits that come up in a search are mostly one of a few types, and the NIAID guidelines name them. Guideline 12 recommends "not using any of the following nonstandardized tests for the routine evaluation of IgE-mediated FA", and the list includes hair analysis, applied kinesiology, allergen-specific IgG4, cytotoxicity assays, and the electrodermal test sold as Vega. Those are the same names printed on many of the kits sold online.

The IgG kits are the ones that look most scientific, with a proper lab report and colour-coded bars. The AAAAI is plain about them: "Specific IgG testing is not a test for an allergic condition." The same sentence in the Choosing Wisely list puts IgG testing alongside the indiscriminate IgE battery as the two things not to do.

Kit reports tend to end in a long list of foods flagged for the family to act on. For a baby still in the first year of solids, acting on a list like that would have meant real food off the plate on the strength of tests the allergy guidelines leave out.

Our baby's milk allergy was the slow kind: eczema on the cheeks and behind the knees that flared a day or two after dairy, which we only spotted because the meals and the skin were written down against the same dates. The cow's milk allergy symptoms post tells that part.

That kind of allergy is not IgE-mediated, so the tests above are looking for an antibody that is not driving it. The Alder Hey Children's Hospital leaflet on delayed cow's milk protein allergy says it directly: "If delayed cow's milk allergy is suspected, skin prick testing and/ or blood tests are not helpful." Instead, it says, the diagnosis "needs to be confirmed or excluded by starting a trial elimination of all cow's milk protein", from the baby's diet if formula fed or from the mother's diet if breastfed, followed by planned reintroduction.

Reading that leaflet explained why our diagnosis leaned so heavily on six weeks of written dates rather than on a lab. A home kit would have had nothing useful to say about it.

The NIAID guidelines recommend "a detailed medical history to help focus the evaluation" of a suspected food allergy, and the AAAAI wants testing "guided by the patient's history". For a baby, most of that history is whatever the parents wrote down.

A useful record for this has the food and the form it came in (smooth peanut butter thinned in porridge, egg baked into a muffin), roughly how much, and the time. Beside it goes any symptom with its start time, where it was on the body, and a daylight photo. Foods that have gone down every week with nothing to note belong in the record too, because they are part of the history as much as the reactions are. Building a food diary for the paediatrician covers the layout.

With that in hand, the list of foods worth testing is usually short, sometimes a single food or none at all. Everything eaten without trouble has already answered the question a test would have asked.