One of the more common questions I am asked in clinic regards oral challenge testing for food allergy. The gold standard of food allergy testing is the oral challenge. Eating is the best test, as false positive allergy testing to foods occurs more often than we would like. The majority of the time, an oral challenge for food allergy is not necessary, though. I will try to take you through my thought process used in determining whether or not to offer an oral challenge to a patient.
The decision on whether or not to do an oral challenge involves two things. The first is the history. What is the history of the reaction to food? Was the timing of the reaction consistent with the food allergy reaction? Were the symptoms of the reaction (hives, wheezing, swelling, G.I. symptoms), consistent with a food allergy reaction? Did the patient use to eat the food without any problems then all of a sudden one time they had some symptoms that were not super typical for a food allergy reaction? Was the duration of a reaction consistent with a food allergy reaction (sometimes hives caused by a virus are falsely attributed to a food allergy).
The next consideration is what the allergy testing shows. Through some very nice research that has been done in the past, we have what are called decision points for oral challenges. For this reason, I tend to like using the Immunocap specific IgE concentration to the food in the blood to help decide on the necessity for the oral challenge. For a given food, based upon research, we have negative and positive predictive values for many of the most allergenic foods. For instance, a peanut specific IgE concentration of over 15 ku/L essentially shows that the patient has a 100% chance of reacting to peanut. For wheat, you would need to get to a level of 26 ku/L for a 75% chance that the patient would have a reaction. So as you can see it is different for each food. It is super important to note that the number does not denote the severity of the reaction, only the likelihood of a reaction.
While these levels are really helpful to have, there is some nuance to this. Some newer research shows that for kids with very high total IgE levels, such as those with atopic dermatitis or eczema, many have falsely elevated specific IgE concentrations to foods that they can tolerate without issue. There is some even newer research showing that a ratio of the specific IgE concentration of the food to the total IgE can be helpful to use in addition to the traditional decision point for oral challenges, and for these kids with atopic dermatitis it can be useful to have this value as well.
There are three basic reasons to do an oral challenge in my mind. The first two are pretty straightforward and commonly encountered in the practice of allergy. The first reason is that someone comes in, usually a small child, who has a history of a positive allergy test to a food that was found incidentally. Usually, the positive test to the food was part of an allergy panel that was drawn for other reasons. If the child has previously been eating the food, I tell the parent that eating is the best test and that clearly the lab was a false positive. For these patients, an oral challenge is not necessary since they are already safely eating the food. But, sometimes there is no ingestion history one way or the other. At that point, we refer to the decision point data on the likelihood of an allergic reaction taking place. Based upon the negative or positive predictive values associated with the number, we decide whether or not to perform an oral challenge to see if the food can safely be consumed or to continue avoidance, consider the child food allergic and provide a food allergy action plan.
The second reason to do an oral challenge is where there is clearly a history of food allergy with corresponding positive allergy testing. But, several years have passed since a reaction has occurred, and generally there has also been a decline in the specific IgE concentration of the food in the blood to the point where we think the child might have outgrown the food allergy. This almost always happens in milk and egg allergy, thankfully. Also, about 20% of peanut allergic toddlers and 10% of tree nut allergic toddlers will "outgrow" these allergies over time so we follow these levels as well in the hope that this can take place. When the level has fallen to a decision point with acceptable risk, we proceed with the oral challenge.
The third reason to do an oral challenge is the least common. I call these oral challenges threshold challenges. This challenge generally happens when there is no history of a reaction, but the specific IgE level to the food essentially shows that there is a very high chance of reacting. Often times, the parent is skeptical that a food allergy actually exists because there has never been a reaction and they have not been particularly careful about avoiding the food (although there has also not been any overt ingestion). In these scenarios, I offer an oral challenge to essentially see what threshold of allergen will cause symptoms in the patient. It is a useful tool to show parents or other caregivers the reality of the situation in a controlled environment. It also can be helpful in that we allow someone who is skeptical of his/her child's food allergy to see a food allergy action plan in action and to become more comfortable with it.
This is how an oral challenge works. We start with a very small amount of the food in question. The starting dose is generally based upon the history of reactions and the likelihood of reaction. It can be as little as a few milligrams or as much as 100 mg of the food in question. Then, the dose of the food is essentially doubled every 15 to 20 minutes until either a full serving of the food is reached, or the first sign of an allergic reaction is noted. Generally, the symptoms of an allergic reaction are more mild than what is seen in real life as we try and use the least amount of food necessary to cause symptoms and treat the reaction immediately. The most common reaction is vomiting and G.I. symptoms. The second most common type of reaction is hives. Occasionally there can be wheezing. Oral challenges historically are very safe. To date, to my knowledge, there has never been a death associated with an oral challenge in an allergy clinic setting.
Hopefully, my explanation of oral challenges hasn't made things murkier for you. But as I hope you can see, an oral ingestion challenge is an important diagnostic skill in an allergist's toolkit.