Diagnosis of food allergy: History, examination and in vivo and in vitro tests
One cannot depend on one single test to diagnose food allergy. A detailed history is an essential initial step in cases of suspected food allergy. Aspects of the history should be gathered separately for each food being considered, as a patient may experience different types of reactions with various foods, each of which requires individual diagnostic and management strategies. History alone is not diagnostic and additional measures of sensitisation or food challenges are often required. In suspected immunoglobulin E (IgE)-mediated allergy, skin-prick tests (SPTs) and/or measurement of serum specific IgE antibodies (ImmunoCAP) to suspected foods is used to prove sensitisation. Sensitisation does not, however, confirm clinical food allergy as these tests indicate an immunological response to the specific allergen, but the diagnosis requires a clear correlation between the test result and clinical reaction (by positive history or food challenge). The magnitude of the test result (SPT mean wheal size or ImmunoCAP level in kU/L) correlates with the likelihood of clinical allergy, but not the severity of a reaction. Choice of the allergens tested should be guided by the history, but limited to the lowest necessary number to avoid false-positive results. Tests for sensitisation to foods should not be performed when the history indicates that such foods are tolerated. Ninety-five per cent positive predictive values (where a clinical reaction can be predicted in 95% of cases) have been described for immediate reactions, but may be population specific. There are no validated tests to confirm non-IgE- or mixed IgE- and non-IgE-mediated food allergies. Diagnosis of this group of allergies depends on elimination of the suspected food, clearance of symptoms, and recurrence of symptoms on re-introduction of the food.