The best food allergy test is an oral challenge

Parents frequently ask questions about food allergy. Some ask because they have a family member with an allergy and wonder if there’s a way to tell if Johnny has it too, while some without any family history ask if foods should be avoided to prevent allergy. Others have children with rashes or stomach aches and want to know if food allergies are the cause. Still others have heard about people who have eliminated foods from their child’s diet and have seen improvements and wonder if they too should adjust their child’s foods.

Many foods have potentially negative effects, but not all of these are allergies. Lactose intolerance, where the stomach gets bloated and gassy often accompanied by diarrhea after eating dairy products, is an example of a food problem that wreaks havoc with the intestinal tract but isn’t an allergy. Another dietary source known to cause significant symptoms but is not a classic allergy is fructose.

True food allergies are subdivided into two distinct categories: protein-based allergies and IgE-mediated allergies.

Protein-based allergies are typified by milk protein allergies seen primarily in infants who, upon exposure to casein (the protein in milk and other dairy products) develop bloating, intestinal inflammation and blood in their stool. Another protein-based allergy is gluten allergy, which is better known as Celiac disease. In this case, exposure to the gluten protein in the intestinal tract results in local symptoms of bloating and poor absorption of nutrients, but can also cause an immune-mediated response that results in other symptoms such as poor growth, recurring mouth ulcers, thyroid disease and even type II diabetes. Although celiac disease can be screened for using a blood test, other protein-based food allergies cannot.

Most food allergies, though, are IgE mediated, which means that the immune system overreacts to the food (or cat hair or whatever) in a predictable way with the release of histamines that cause wheezing, hives and/or vomiting. Any child with generalized hives after eating a food may indeed have a food allergy and should be assessed. Allergy skin and blood tests are useful to a point, but are not proof one way or the other about whether a food is truly allergenic for a given person. Indeed, a person may test positive for foods he eats without difficulty and have a negative test for a food that, when eaten, causes vomiting every time.

False negative tests (where you have symptoms of allergy but the test is negative) are less common than false positive tests (where the test is positive but you tolerate the food just fine), which occur pretty frequently. But having the data can still give some guidance. If a blood test or prick skin test is hugely positive (with a very high number or large wheal — a raised area of skin indicative of an allergic reaction), chances are an oral challenge with the food is going to result in symptoms.

The reality, though, is that the only truly reliable test to determine if an IgE mediated food allergy is present is to give that food in the form of an oral challenge, where a small amount of the food is ingested and then the child is observed for any evidence of allergic reaction. The vast majority of the time an oral challenge is only needed in the case where the supposed reaction was mild (just a small rash) or when more than one food could have been the cause. If your child had hives and vomited profusely following peanut butter ingestion, it’s clear what is going on and the only reason to do blood tests is to have a sense of how high the level is to follow it over time. In the case of vomiting and hives, there is no reason to do an oral challenge.

Many times, though, parents will come in with photos of a mild rash, often localized to the face or places the food or drool could have touched and want to know if this is indicative of a food allergy. Since contact rashes can occur with lots of foods and aren’t allergic in nature, my rule of thumb is that if the rash is only in areas the food or drool could touch this isn’t allergy and you can go ahead and continue offering the food. When the rash is a bit more generalized than that but no vomiting, coughing or wheezing was present, an oral challenge may be the best way to to determine if the child is allergic to the food. Oral challenges should occur in an office setting where trained personnel are available to manage any potential reaction with benadryl or epinephrine if needed. I would also suggest putting some petroleum jelly on the cheeks, chin and upper chest of the child to limit the contact exposure and decrease the likelihood of misinterpreting a rash following consumption. Within a short time after ingestion (no longer than 4 hours), allergic symptoms will appear if the offending food is a problem.

Sometimes children inadvertently ingest a food at home that they are allergic to (an inadvertent oral challenge) and if no allergic symptoms occur, then continuing to slowly integrate the previously offensive food into the diet at home makes sense. Since many food allergies are outgrown over time, an inadvertent challenge can be useful.

Oral challenges are potentially risky and ideally should be done at an allergist’s office to ensure the safety of your child. But if the initial symptoms that raised the question of food allergy are mild or unlikely to be due to food, an oral challenge in your pediatrician’s office to rule out a specific food as a problem may make sense.