Diagnosis of food allergy

Diagnosis

• food allergy can be classified into IgE-mediated and non-IgE-mediated allergy. IgE-mediated reactions are acute and frequently have a rapid onset. Non-IgE-mediated reactions are generally characterised by delayed and nonacute reactions (1)
• diagnosis of food allergy depends on specific tests and elimination diets and food challenges
• based on a good clinical history, specific IgE tests, elimination diets and food challenges
• if an immediate (IgE mediated allergy is suspected) then radioallergosorbent test (RAST) for food specific IgE antibodies (2,3)
• skin prick tests are a secondary care based investigation (2,3)
  • there is only a 50% positive predictive value associated with a positive test result (i.e. 50% of patients testing positive will have a falsely positive test)
  • a negative test however has a high negative predictive value - a negative test will exclude immediate food allergy in > 95% of cases
• an oral challenge test is a secondary care based investigation
  • may be indicated if there is discrepancy between clinical history and specific IgE and skin prick tests or when wishing to assess whether or not a child has 'outgrown' their food allergy

IgE-mediated food allergy (1)

• based on the results of the allergy-focused clinical history, if IgE mediated allergy is suspected, offer the child or young person a skin prick test and/or blood tests for specific IgE antibodies to the suspected foods and likely co-allergens
  • tests should only be undertaken by healthcare professionals with the appropriate competencies to select, perform and interpret them
• skin prick tests should only be undertaken where there are facilities to deal with an anaphylactic reaction - considered a secondary care investigation (2,3)
• choose between a skin prick test and a specific IgE antibody blood test based on: the results of the allergy-focused clinical history and whether the test is suitable for, safe for and acceptable to the child or young person (or their parent or carer) and the available competencies of the healthcare professional to undertake the test and interpret the results
• do not carry out allergy testing without first taking an allergy-focused clinical history. Interpret the results of tests in the context of information from the allergy-focused clinical history
• do not use atopy patch testing or oral food challenges to diagnose IgE-mediated food allergy in primary care or community settings

Non-IgE-mediated food allergy (1)

• based on the results of the allergy-focused clinical history, if non- IgE-mediated food allergy is suspected, trial elimination of the suspected allergen (normally for between 2-6 weeks) and reintroduce after the trial. Seek advice from a dietitian with appropriate competencies, about nutritional adequacies, timings of elimination and reintroduction, and follow-up
  
  

Reference:

• 1) NICE (February 2011).Food allergy in children and young people Diagnosis and assessment of food allergy in children and young people in primary care and community settings
• 2) Pulse (March 26th 2005): 50-57.
• 3)Sicherer SH.Food allergy. The Lancet 2002; 360(9334):701-710