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Patients often know what they are allergic to e.g. fruit or shellfish. The reaction is one of an immediate Type 1 hypersensitivity. Avoidance of the trigger is recommended and to carry a supply of non-sedating antihistamine
Many physical factors may induce urticaria e.g. cold, heat, pressure, water, cholinergic, solar, vibration. Dermographism can easily be demonstrated by firm stroking of the skin with the thumbnail and whealing resulting in five to ten minutes.
By definition this is urticaria occurring on most days for over 6 weeks. There is unlikely to be an identifiable allergic cause and there is increasing evidence that chronic urticaria is auto-immune in many patients.
Advise patients to avoid all aspirin containing drugs, which may aggravate chronic urticaria. There is no need to refer patients with chronic urticaria for routine testing. For persistent disease check FBC, ESR and TFTs. Check specific IgE to allergens only if strongly suspected from history.
The histamine receptor is polymorphic and patients respond differently to different antihistamines. It is worth trying others if the above choices are unsuccessful. It is more logical to choose others which are structurally distinct such as one of the older sedating antihistamines or fexofenadine (NB dose in urticaria is 180mg daily).
It is common practice for dermatologists to titrate non-sedating antihistamines to 2-3 times the licensed doses if tolerated.
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