Fourteen year old Hanna was at the mall with friends when she discovered she was allergic to macadamia nut. This fact became clear when she nearly died. She had only taken a few bites of the ice cream with macadamia nut from Cold Stone Creamery. She had eaten a variety of nuts on numerous occasions without problems, her favorites being almond and hazelnut, but his was her first time to try this macadamia nut.
By the time paramedics had arrived, she was covered in hives, felt dizzy, had vomited, and could not stand
Immediately her mouth began to burn and after a few minutes her face turned bright red. She felt itchy all over, her eye lids and lips began to swell and she had a hard time breathing. By the time paramedics had arrived, she was covered in hives, felt dizzy, had vomited, and could not stand. The paramedics did not waist anytime: A syringe filled with epinephrine was jabbed into her upper leg, an IV was started and connected to a larger bag of fluid and she was taken to the nearest hospital. When she arrived at the emergency room she was feeling better, the hives were clearing and she could breath easier. She was able to go home after several hours.
Trip to the Allergy Office
She was seen in our office the next week for allergy testing. A small drop of macadamia nut extract was placed on the skin of her back and gently pricked. After 20 minutes the test was read by measuring the size of any swelling at the test site, the “wheal” as well as any redness or erythema. The size of the test wheal is compared with a negative (saline) and a positive (histamine) control. A wheal that is 3-4 mm larger than the negative control is a positive test. Hanna’s macadamia test was over 20 mm!
For Hanna, the diagnosis of allergy to macadamia nut is clear. She has a compelling history of typical allergy symptoms appearing within minutes of exposure and a strongly positive result on allergy skin testing to the allergen in question. The recommendation is also clear. No macadamia nuts…EVER. She will also need to carry an EpiPen in case of accidental exposure. Having it nearby could be life saving.
Now let’s back up a notch.
Let’s suppose Hanna stayed home from the mall that day to study for a test and mom decided to make an appointment with an allergist to talk about Hanna’s itchy eyes and sneezing during the spring. In this alternate universe, Hanna has never eaten macadamia nut. During the visit mom mentioned that Hanna complained of itching of the mouth with almonds and hazelnuts when she was a child but is now eating both nuts fairly regularly without problems. She would like to check for tree nut allergy though, “just to be sure”.
Allergy testing is completed and the results show strongly positive reactions to spring tree and weed pollen. Testing to the tree nuts showed an 8 mm reaction to almond, 6 mm reaction to Hazelnut, both moderately positive. Macadamia nut showed a 20 mm skin test reaction.
What is the diagnosis? Is Hanna allergic to almond and hazelnut?
To answer this important question, we need to start with how we define “allergy”. The definition of allergy has two parts:
- A history of symptoms (rash, abdominal pain, breathing difficulty, etc.) on exposure to an allergen
- A positive result to the suspected allergen on a skin or blood test for allergy
We like to have both to make a diagnosis of allergy. Even though Hanna’s allergy test is positive to almond and hazelnut, she has eaten both frequently without problems. By definition therefore, she is not allergic to almond or hazelnut.
“Why are the almond and hazelnut allergy tests positive if she is not allergic.”
There are a number of possibilities but it is most likely that the positive skin test reaction is the result of cross reaction between proteins in the tree nut and very similar proteins in the tree pollen that she is sensitive too. (To learn more about false positive allergy tests, go here.)
Although Hanna understands this explanation (“sort of”) she is still concerned about eating almond and hazelnut now that she knows that the allergy test was positive. She asks if there is another test that will prove she is not allergic.
In fact, there is….
The Double Blind, Placebo Controlled Food Challenge
The double blind, placebo controlled food challenge is considered the gold standard for diagnosing food allergy. In this test two foods are prepared, the test food and a placebo food made to look and taste like the test food. Neither the patient nor those conducting the test know which is the true food and which is the placebo until after the test is completed. The use of the placebo removes the chance that symptoms may be related to the patient (or test giver) having symptoms caused by anxiety.
Because of difficulty associated with developing a suitable placebo for every test food, double blind, placebo controlled food challenges are infrequently done. In stead, open, graduated challenges of the suspected food have become more popular.
Graduated Food Challenge
In this test, the patient is given a very tiny amount of the suspected food while being closely monitored for any signs of a reaction. If there are no problems after 15-30 minutes, the dose of the food is increased. This process is continued until a target dose is achieved or until any objective symptoms suggesting a reaction occur.
Allergy testing is always done prior to a food challenge. If the allergy test is negative or if the patient has a clear history of having eaten the food without problems, the chance of passing a food challenge is very high. A food challenge is not recommended in someone who has a clear history of a severe allergic reaction to a food and has a positive allergy test. Again, this patient is allergic by definition and a food challenge is not needed.
What about the macadamia nut?
More challenging is the question of conducting a food challenge in a patient who shows a strongly positive result on an allergy test and who has never eaten the food. This is the case with Hanna and the macadamia nut. Because she has never eaten macadamia nut, there is insufficient information to make a diagnosis of allergy. On the other hand, the likelihood that she would fail a food challenge test is high because of the strongly positive skin test result. In this case, the safest recommendation would be for her to avoid macadamia nut. Although not perfect, cut off values for the size of skin test reaction likely to result in a failed food challenge have been published.
Delayed introduction of allergenic foods because of allergy testing may cause the problem that well-meaning parents are trying to prevent.
There is increasing evidence that early introduction of allergenic foods to infants is associated with a decrease in food allergy. For this reason, the unintended consequence of allergy testing of children to foods they have never eaten in hopes that it will be prevent them from having an allergic reaction, is that parents will be afraid to introduce foods that show positive on the test. Delayed introduction of allergenic foods because of allergy testing may therefore cause the problem that well-meaning parents were trying to prevent.