In contrast to in vitro tests, skin tests are dependent on the skin status, and are influenced by medication and the way the test is performed. There is a small, however finite, risk of systemic reactions induced by skin tests. In vitro testing with Pharmacia CAP System RAST testing. About half of all IgE mediated reactions to food occur more than one hour after ingestion. In these cases the patient is rarely certain about the reason for the reaction.
Immediate reactions are typically mouth irritation, swelling or vomiting.
In these cases the patient is usually aware of the foods responsible. This must in any event be confirmed. If the reaction has been severe such as an anaphylaxis, it is important to complete an IgE determination to correctly identify the allergen. In such cases a food challenge or skin test could be dangerous. The search for the causative allergen may be laborious because the patient is usually unaware of what was eaten and when.