Food-allergic consumers can suffer allergic reactions from trace quantities of hidden allergenic residues. Immunochemical assays were used to determine if food-allergic consumers could successfully identify the food products that elicited their (or their family member's) allergic reactions.

177 food samples suspected of causing allergic reactions by consumers and/or allergists were submitted for testing from November of 1998 to August 2003. The samples were analyzed by various commercial and in-house immunochemical assays for the hidden presence of peanut, almond, walnut, wheat, soy, egg and milk (casein and whey). 392 tests were performed on the 177 products based upon the suspicions of the consumers or allergists.

Of the 177 samples, 25% were positive for one or more hidden allergens including 28 of 72 for casein (milk), 2 of 9 for whey (milk), 8 of 190 for peanut, 2 of 47 for almond, 2 of 29 for walnut, 2 of 28 for egg, 0 of 7 for soybean, and 0 of 10 for wheat gluten. Levels of hidden food allergens in these products ranged from 2 ppm to 215,000 ppm.

Consumers experiencing allergic reactions were only able to identify the causative food product in 25% of the cases. Immunochemical analyses can be useful in confirming consumer suspicions regarding the causative product. Some of these implicated food products may have contained residues of other allergens for which testing is not yet available e.g. other tree nuts.