Do skin prick testing for common food allergens at the time of the initial visit. Ensure the antihistamines were stopped at least 5 days prior to testing and there is no skin rash in the area to be tested. Do not do RAST initially because skin prick testing is both more sensitive and clinically relevant.
If the skin prick test is positive, check RAST in one year and yearly after that. Do not consider a food challenge unless the specific IgE level is less than 1. The lowest level is less than 0.1 but this is a "false basement," i.e. a lab "zero" is not the same as clinical "zero." A patient may have a specific IgE level of less than 0.1 and still react to peanuts on exposure.
The reported IgE values on RAST range from 1 to 100. If a patient has a history of allergic reaction to peanuts and IgE level is 5, he has a 100% chance of clinical reactivity. If the IgE level is 1, there is a 40% chance of clinical reactivity. If the IgE level is less than 0.1, there is a 25% chance of clinical reactivity. There is no clinical "zero" from the RAST which can guarantee "zero" chance for clinical reactivity.
Relative sensitivity of RAST, skin prick testing and intradermal testing. Image source: Adapted from Dr. Hopp, Creighton University Division of Allergy & Immunology, used with permission.
There are predetermined levels of specific IgE on RAST below which a food challenge can be attempted. Those levels are shown in the grid below:
Levels of specific IgE on RAST below which a food challenge can be attempted. Image source: Dr. Hopp, Creighton University Division of Allergy & Immunology, used with permission.
Read more here:
Food Allergy in a Toddler
When to Attempt a Food Challenge in Nut Allergy?
Image source: Wikipedia, public domain.