The diagnosis of food allergy depends primarily on a detailed history as outlined in the algorithm, looking for supporting features such as reactions to common cause foods, clinical signs, symptoms and timing.
There is no symptom or sign that is absolutely specific to allergic disease.
The most important history feature is the timing of onset of symptoms after contact with the food; IgE mediated reactions usually occur within minutes and by definition within 2 hours of contact.
Very small amounts of food can elicit significant IgE mediated reactions.
Delayed or non IgE mediated food allergy is a well recognised entity although there is currently no validated test to confirm it.
A small proportion of children with significant colic, gastro-oesophageal reflux or constipation in infancy have delayed food allergy.