Explanation to Queston 3
The diagnosis of food allergy is established through clinical history and oral challenge. Skin tests and serum specific IgE provide evidence of sensitization and help determine when a challenge is appropriate.
Patients with large wheals from skin prick testing (SPT) or with high serum specific IgE are likely to have food allergy.For certain foods (e.g., cow’s milk, egg, peanut), cutoff values associated with high likelihood for having a reaction have been published. The cutoff value for IgE and wheal diameter is different for each food and is also dependent on the age of the child. The cutoff value is the concentration of specific IgE for a particular food allergen that is 90% predictive of a clinical reaction to the food. Increasing levels of specific IgE and wheal response to SPT beyond the cut-off level correlate with an increased likelihood of having clinical food allergy but not necessarily with reaction severity. For example, established cutoff values for serum specific IgE testing include:
Egg (child 3-17 yrs): 7 kU/L
Egg (child < 2 yrs): 2 kU/L
Cow’s milk (child 3-17 yrs): 15 kU/L
Cow’s milk (child < 2 yrs): 5 kU/L
Peanut (child < 18 yrs): 14 kU/L
Fish (child < 18 yrs): 20 kU/L
Jack is therefore likely to be truly allergic to peanut since his IgE value is over the 14 kU/L cutoff value. Note that Jack was not tested for egg or milk allergy as his parents reported regular consumption of these foods without symptoms of food allergy. If a child is already consuming a food without experiencing a clinical reaction, SPT and IgE testing is not warranted. If for whatever reason testing is conducted and positive results are observed, it is important to stress to the caregiver that the food continue to be consumed on a regular basis. Because testing indicates that the child is sensitized to the allergen, a prolonged absence of the food from the diet could lead to clinical symptoms upon re-exposure.
While there is no cross reaction between peanuts and tree nuts, children allergic to one have an increased risk of allergy to the other. Since peanuts and tree nuts are often processed together, it is often advisable to strictly avoid both peanuts and tree nuts to avoid exposure through cross-contamination.
The correct answer is D, Jack should continue to avoid peanuts and tree nuts.
Sources:
1. Sampson H. Update on food allergy. J Allergy Clin Immunol 2004; 113:805-19.
2. American College of Allergy, Asthma, & Immunology. Food allergy: a practice parameter. Ann Allerg Asthma Immunol 2006; 96:S1-68
3. Clark AT, Ewan PW. The development and progression of allergy to multiple nuts at different ages. Pediatr Allergy Immunol 2005; 16:507–11.
