Allergy Testing in Primary Care

Darren is 28 years old, and you’ve seen him a few times over the years; he’s fit and well generally, but has developed some intermittent GI symptoms over the last few years, which at times seem worse after certain foods. He’s fairly clear about his agenda - ‘I think I’ve got allergies and need some tests’. Some gentle prompting about what has led him to see you today reveals that he’s been doing some ‘research’ on TikTok and thinks his symptoms warrant allergy testing. Hmmm.

Allergies, and testing thereof, seem to have become ubiquitous, as a brief trip down the Internet doom hole confirmed this morning. Private tests are advertised everywhere, including new DNA genetic tests to give you ‘insight’ into your genetic profile (oh dear), and influencers are promoting all manner of theories and suggestions on tests on SnapTokInstaBook.

So it was a great relief when I came across the new Primary Care Guideline from the British Society for Allergy and Clinic Immunology (BSACI) on allergy testing in Primary Care. It gives an excellent evidence based summary of what we should (and in many cases what we shouldn’t) be doing when it comes to allergy testing. 

For clarity this guideline reviews testing for IgE-mediated hypersensitivity, and references to ‘allergy’ in this guideline are synonymous with immediate/type 1 (IgE-mediated) hypersensitivity, and ‘testing’ refers to allergy specific IgE (ASIgE) tests by skin prick and bloods - given most skin prick testing is done in specialist clinics, this guideline essential boils down to when we should, and should not, do blood IgE testing in Primary Care. The key principle is summarised in the introduction ‘Testing should be strictly guided by the clinical history and ASIgE blood testing should only be requested when there is a compatible clinical history. There is no role for “screening” with multiple ASIgE tests’.

We summarise the guideline in the attached KISS, but a few key points and reminders are worth highlighting: 

• IgE-mediated allergy is triggered by a variety of allergens (foods, inhalants, drugs, insect stings), and is an acquired condition whereby B lymphocytes produce IgE after a sensitisation event. IgE can then be detected in the blood bound to mast cells, which then release various mediators (e.g. histamine, mast cell tryptase) on re-exposure to the allergen.
• Key points on the clinical features:
  • The point at which sensitisation happens is very variable, so prior tolerance does not exclude allergy.
  • Symptoms occur within minutes/max 1 hour of exposure and include cutaneous features (urticaria, angioedema or erythema), wheeze, vomiting and diarrhoea, hypotension. The guideline notes ‘Isolated delayed (>1 hr) GI features are generally not consistent with allergy and patients should not be tested for ASIgE…’
  • Symptoms are REPRODUCIBLE and occur with EVERY subsequent exposure.
  • Symptoms do NOT occur without exposure to the allergen.
• Key principles:
  • Allergy is a CLINICAL diagnosis, supported by testing (not the other way round…).
  • Testing should be strictly guided by the clinical history, and only done with a compatible history (see STAR principles below).
  • There is NO role for ‘screening’ with multiple IgE tests.
  • Limit tests to relevant triggers only:
    • If applying the STAR principles, this should rarely be more than 5 individual allergy-specific IgE tests; bundles of tests, e.g. food mixes, are not helpful.
  • Only test if it will lead to a change in management, e.g. if the allergen can't be avoided (such as pollen), it's questionable if testing will help.
  • Urticaria (on it’s own) is rarely due to allergy; the majority of urticaria is spontaneous and benign in nature, and unless there are clear and consistent triggers, allergy testing is NOT indicated.