While prick skin testing to foods determines the presence of food allergen- specific IgE bound to specific receptors on the surface of mast cells in a patient’s skin, laboratory assays have been developed that determine the presence and amount of food allergen-specific IgE circulating unbound in the serum.

These assays are particularly useful when medications that impact skin testing cannot be discontinued, widespread skin disease is present, or some other circumstance, such as unavailability of an extract, precludes skin testing.

In addition, these assays are used to further investigate situations involving a suggestive history but a negative skin test, or to determine the level of circulating food-specific IgE in the patient with a positive skin test, to aid in deciding whether a food challenge to document tolerance of the food is indicated.

Although the sensitivity of skin testing and selected immunoassays is comparable, in the patient with a highly suggestive history and a negative immunoassay, if not already obtained, the performance of a skin test is advised before ingestion of the food is encouraged.

Following the levels of food allergen-specific IgE in an individual patient over time can be used to evaluate whether sensitization to the food is increasing, stable, or waning. Food allergen-specific IgE levels aid in predicting the likelihood of a reaction, but do not predict reaction severity.

One of the most widely used and evaluated immunoassays is the ImmunoCap radioallergosorbent test (CAP RAST) system, which measures the amount of circulating allergen-specific IgE in the serum in kilounits of allergen- specific IgE per liter (kUA/L).

In 1997, in a retrospective study analyzing the sera of food allergic children in light of highly suggestive histories or the results of food challenges, Sampson and Ho reported predictive threshold levels for several of the commonly allergenic foods, including milk, egg, peanut, and fish.

Patients with values higher than the calculated threshold values had a 95% likelihood of reacting upon ingestion of the food. In a subsequent prospective study, Sampson evaluated 100 children by history, oral food challenges, and measure of allergen-specific IgE levels to egg, milk, peanut, soy, wheat, and fish.

This study was undertaken to determine if the 95% predictive decision points for egg, milk, peanut, and fish determined in the previous study were accurate. Using the previously defined predictive decision points, more than 95% of the food allergies to these foods in this population of patients were correctly identified.

These diagnostic decision points have been further investigated and refined, and their use in clinical settings has significantly reduced the need for food challenges. CAP RAST levels, above which 95% or more of children would be expected to react, have been reported for several of the major food allergens.

Children less than 2 years of age with a CAP RAST to egg of greater 2 kUA/L or a CAP RAST to milk of greater than 5 kUA/L, have a greater than 95% chance of reacting on food challenges. For older children the decision points for foods commonly causing allergic reactions are as follows: egg 7 kUA/L, milk 15 kUA/L, peanut 14 kUA/L, tree nuts approximately 15 kUA/L, and fish 20 kUA/L.

However, individual patient CAP RAST results often fall in an indeterminate zone below the diagnostic threshold but above the value predicting tolerance. In addition, because these decision points have been determined for a relatively small number of foods, threshold values for the suspected food may not have been established.

As a result of these and other nuances, food challenges remain an important tool for documenting the association between the ingestion of a suspected food and the onset of symptoms.