adult food allergy
DIAGNOSIS OF IgE-MEDIATED FOOD ALLERGY IN ADULTS Adult food allergy presents with a range of complex symptoms. A good diagnostic work-up is required, followed by appropriate dietetic advice.
Tak Chin Specialist Registrar in Allergy Medicine, Southampton General Hospital
Carina Venter PhD, RD Senior Lecturer, University of Portsmouth
Dr Tak Chin is a Specialist Registrar in Allergy Medicine at Southampton General Hospital
As well as being a Senior Lecturer at the University of Portsmouth, Carina is an Allergy Specialist Dietitian with The David Hide Asthma and Allergy Research Centre on the Isle of Wight. She is also a Food Allergy Module Leader at the University of Southampton.
In clinical practice, the offending food(s) causing IgE-mediated food allergy can often be identified by taking a careful clinical history. Important elements of the history include the type of food involved, quantity of food ingested, time between ingestion to reaction, symptoms/signs of the reaction, other occasions when similar reactions occurred, time since last reaction and other factors involved (e.g. exercise). In complex cases, a food-symptom diary may be required as an adjunct to the history. The usual first-line diagnostic tests, routinely used in conjunction with the clinical history, to establish the diagnosis of IgE-mediated food allergy, are skin prick tests (SPT) (which can be performed with commercially-prepared standardised solutions of food allergen extracts, or with fresh foods (prick-toprick (PTP) testing) and serum specific IgE (sIgE) to food allergens. SPT is generally preferred as it offers a quick and reproducible method for detecting IgE sensitisation. Although it is a safe procedure, severe and fatal anaphylactic reactions have been reported in those with highly severe allergies (1). It is, therefore, recommended that SPT is performed by experienced personnel in an appropriate setting with access to emergency medications and equipment. sIgE offers an in vitro method for quantifying IgE sensitisation and may be useful when SPT is not possible (e.g. limited skin surface for SPT, dermatographism, suppressed skin reactivity due to antihistamines, needlephobia). Sensitivity and specificity varies depending on the food, as well as other factors (e.g. allergen extract, commercial test system, age of the patient). In general, a negative SPT result has an excellent nega-
tive predictive value, while a positive SPT result only indicates the possibility of symptomatic IgE-mediated food allergy. One exception to this is IgE-mediated food allergy to certain fruits/vegetables (particularly those associated with pollen-food allergy syndromes), which may sometimes not be detected using commercially-prepared extracts/reagents due to the labile nature of the allergenic epitope involved. For some of the common major food allergens (e.g. hen’s egg, cows’ milk, peanut), decision points based on SPT size and also sIgE titre cut-off values have been described which have a >95 percent positive predictive value for IgE-mediated food allergy. However, decision points have not been successfully established for other major food allergens (e.g. soy, wheat) and the less common food allergens. A potential issue with both SPT and sIgE is the possibility of cross-reactivity to common allergenic epitopes in related foods, unrelated foods and pollens which may result in false positives to food allergens that are not clinically relevant. For certain foods - componentresolved diagnostics (CRD) - which involves the measurement of sIgE against purified individual proteins within the food - may be useful in determining the risk of severe allergic reactions. Numerous food proteins have been identified and knowledge of their allergenic significance continues to evolve. In general, sensitisation to seed storage proteins and non-specific lipid transfer proteins (nsLTP) is usually associated with a higher risk of severe allergic reactions. In contrast, sensitisation to Bet v 1-related PR10 proteins is usually associated with mild allergic reactions to fruits and vegetables NHDmag.com February 2015 - Issue 101
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