Comparative evaluation between two immunoenzymatic techniques (FAST and Phadezym) and the Phadebas RAST in food allergy

The diagnosis of food IgE-dependent hypersensitivity is based on the demonstration of specific IgE, completed by provocation tests. Two immunoenzymatic techniques, the Phadezym and the FAST, are compared with the Phadebas RAST, in 86 sera (23 controls, 28 from patients with a reported food allergy and 35 with a positive RAST to a food allergen). The within-run variation coefficient of class 0-2 sera was 9% for the Phadebas RAST, and higher than 20% for the Phadezym and the FAST. It was in order of 8.7%, 9.4% and of 17.6% respectively for Phadebas RAST, Phadezym and FAST when estimated with class 3-4 sera. The specificity was higher than 95% for the three techniques. The sensitivity was 75% for Phadebas and 43% for Phadezym and FAST. The FAST test is much less sensitive with allergens of vegetal origin than those of animal origin (P less than 0.01). This work indicates the high percentage of false negative results of immunoenzymatic techniques when food extracts are tested. This could be explained either by an enzyme-substrate reaction or by a non-specific inhibition of the enzyme linked to the anti-IgE IgG.