The diagnostic approach to allergic food reactions comprises three steps. The first step includes the medical history, physical examination, and family allergy background. On the basis of symptoms and timing of the reaction, the physician attempts to identify the suspected food and to determine whether the reaction is likely to involve an immunologic mechanism. This first step is absolutely necessary to decide on the subsequent diagnostic tests to be performed. The second step includes allergy skin tests and in vitro assays, which can confirm a sensitization to the food. However, for the conclusive diagnosis of a food allergy testing it is necessary to demonstrate, in a third step, by an oral challenge, that the food to which a sensitisation has been found is responsible for the patient’s symptoms.Skin prick tests are most frequently used as the first test to screen for specific IgE to foods. Skin prick tests are not uncomfortable for the patient, they are easily performed, quick (the result is available in 15 minutes), safe and cheap. For these reasons they are the method of choice to demonstrate an IgE response to foods. In vitro tests are recommended in patients with extensive skin disease, dermographism, who cannot discontinue antihistamines, or with a history of an extreme sensitivity.
The diagnostic accuracy of skin and in vitro tests depends on the quality of the food allergen extracts. In contrast to aeroallergens, food allergen extracts have not been standardized. The performance characteristics of tests for egg, milk, peanut, fish, wheat and soy have been extensively studied, particularly in children with atopic dermatitis. Skin prick tests and CAP to egg, milk, peanut and fish are comparable, with excellent sensitivity and negative predictive accuracy (most >90%), but poor specificity and positive predictive accuracy (50–85%). Therefore, a negative food allergy test with these food extracts is a good method to rule out an IgE-mediated food allergy. In contrast, a positive food allergy testing is only suggestive of the presence of a clinically relevant food allergy, and the final diagnosis should rely on an oral challenge. However, a positive test in a patient who has experienced a systemic reaction after the ingestion of an isolated food should be considered diagnostic. By means of the Pharmacia CAP system, cut-off points with a 95% positive predictive value have been established for egg, peanut, milk and fish. The application of these cut-off points in clinical decisions can reduce the need to perform DBPCFCs in a significant number of patients.
The diagnostic accuracy of Allergy Skin Prick Tests and in vitro IgE assays for fresh fruits and vegetables is poor. The sensitivity of the tests is generally low, for the Rosaceae fruits, presumably due to the liability of the allergens involved. To overcome this problem the prick–prick test has gained popularity. In this test the lancet is plunged several times into the food immediately before pricking the patient’s skin with it. Nowadays, the prick–prick test is the most sensitive test with fresh foods. It is also useful when there are discrepancies between a suggestive medical history and a negative Allergy Skin Prick Tests with a commercial extract, or when a specific food extract is not available. The inconveniences of the prick–prick test are the impossibility of standardization and the dependence on the availability of the fresh food.
Positive Skin Prick Tests and serum-specific IgE to fruits and vegetables are commonly seen in tolerant patients. These false positive results are an expression of IgE cross-reactivity. This is frequently found in pollen allergic patients sensitized to the major birch pollen allergen, to profiling or to carbohydrate determinants of glyco proteins. This latter cross-reactive structure seems to hamper exclusively the specificity of the in vitro assays. To overcome the poor specificity of tests, clinicians have to confirm clinical reactivity or tolerance by means of controlled oral challenges.
