Phadia’s F23 test is used to detect blood levels of the IgE (antibodies) responsible for allergic reactions to crab. Although some individuals may be allergic only to crab, this crustacean has allergens in common with shrimp, lobster, crayfish and crawfish, and multiple seafood allergies are common. The interpretation chart accompanying the result expresses the probability of an allergic reaction to crab (and possibly other seafood), but not necessarily the severity of the reaction. Case history (symptoms, relation to meals, etc.) and clinical examination by the physician are essential for a reliable interpretation of the results.
A negative anti-crab IgE result (< 0.35 kU/L) indicates that crab is probably not responsible for the symptoms reported by the person, but it DOES NOT COMPLETELY RULE OUT THIS POSSIBILITY. Both negative and positive results must be interpreted in light of the case history. In more complex cases, the double-blind oral food challenge under medical supervision remains the definitive technique for confirming the presence or absence of a crab allergy. Unlike milk and egg white allergies, which many children outgrow, the hypersensitivity to crab and other crustaceans tends to persist for life.