Food Allergic Patient Evaluation - Prick Skin Testing

Skin testing to foods is essentially a bioassay that involves introducing miniscule amounts of food allergens into the patient’s epidermis and monitoring the result. If mast cells in the patient’s skin have IgE on their surface specific for the food being tested, binding of the food allergen by these IgE antibodies triggers mast cell degranulation, resulting in histamine release and mediator generation.

The localized mediator release results in the rapid formation of a cutaneous wheal surrounded by an erythematous flare. In the absence of IgE specific for the introduced food allergen, no reaction occurs. Glycerinated commercial food extracts are widely available for skin testing to many common food allergens.

Fresh food extracts, prepared by crushing the fresh food in an aliquot of saline, are also occasionally used. These fresh extracts can be further diluted if there is concern regarding exquisite sensitivity. Alternatively, the ‘‘prick to prick’’ technique can be employed, which involves first pricking the food with the skin test device, immediately followed by pricking the patient’s skin.

These methods are useful when testing for sensitivity to fruits or vegetables containing labile allergens susceptible to degradation during the extraction process used in the preparation of commercial extracts, or when no commercial extract of the suspected food is available.

Fresh extracts can also be prepared to verify the results obtained using a commercial extract when the history is highly suggestive, but the skin test to the commercial extract is negative. In addition, skin testing with freshly prepared extracts can provide a direction for further evaluation.

Skin testing with fresh extracts prepared from foods or sauces from a restaurant meal thought to have caused a reaction can suggest which foods or ingredients are worthy of further investigation. The potential for irritant reactions exists, but can be ruled out by skin testing others not sensitive to the food with the same extract.

After pricking the skin with a lancet or bifurcated needle through which a small drop of food allergen extract is applied to the back or forearm, any resultant wheal and erythema observed at the site after approximately 15 minutes is measured and recorded.

A histamine skin test is applied as a positive control, with a saline skin test serving as the negative control. Based on initial studies performed in children by Bock and colleagues in the late 1970s, a food skin test is defined as positive if a wheal 3 mm in diameter larger than the negative saline control, is observed. Systemic symptoms resulting from prick or puncture skin testing are exceedingly rare.

The use of intradermal skin testing to foods is discouraged, as it has been shown to be less specific and carries a higher risk of systemic reactions. Rather than routinely skin testing to a broad panel of food allergens, skin tests are selected based on foods suggested by the history or limited to the foods considered to be common food allergens.

In general, the positive predictive accuracy of a properly performed food skin test is considered less than 40% when the 3-mm cutoff for defining a positive skin test is used, indicating that many individuals who have a positive skin test to a food can eat that food without ill effects.

However, Sporik and colleagues, evaluating a large cohort of children with a median age of 3 years with skin testing followed by food challenges, were able to calculate skin test diameters to peanut (greater than 8 mm), cow’s milk (greater than 8 mm) and egg (greater than 7 mm) with positive predictive accuracies approaching 95%.

Although their findings cannot be extrapolated to other populations because of potential differences in age, extracts, and technique used, they demonstrate that using a larger wheal diameter to define a positive skin test results in a decrease in sensitivity but an increase in specificity. As with other investigators, they found no correlation between skin test size and the severity of a reaction.

Removing a previously tolerated food from the diet based on skin testing alone is rarely recommended. However, a positive skin test is useful for identifying foods worthy of further investigation and is highly suggestive of a diagnosis in situations when a patient experienced a significant reaction following the ingestion of an isolated food.

Alternatively, the negative predictive accuracy of a properly performed skin test is greater than 95%. Thus, skin testing is a rapid, sensitive, efficient method of ruling out IgE-mediated reactivity to a food when quality extracts are applied using proper technique, and the patient has not taken medications known to interfere with testing.