Are any tests needed?
Tests are often not needed or helpful. Which tests are needed depends on the pattern of when the rash comes.
A symptom diary
A symptom diary may be helpful in working out if there is a specific trigger or cause. If your diary shows you only develop the rash on days when you eat a certain food, for example, it may be obvious what the cause is. A diary might also show if the rash relates to stress, exercise or sunlight. It may help rule out certain causes.
Skin-prick allergy tests
In some cases if it appears to be related to an allergy, tests such as skin-prick allergy tests may be helpful. Tiny quantities of substances which might cause an allergy are placed on your skin in a special order. If the skin reacts, you may be allergic to that substance.
In other cases, blood tests for allergy may be useful. In some people, blood tests may help to point to a cause. These may be general blood tests, or blood tests to look for specific proteins produced by your immune system, called autoantibodies. Where these are found, it is likely that it is your own immune system over-reacting which is causing the rash.
In some people, 'exclusion' or 'challenge' tests may be needed. For example, testing to see if cold or pressure always causes the rash, or excluding certain foods from the diet to see if that stops the rash from coming back.
Further reading and references
BSACI guideline for the management of chronic urticaria and angioedema; British Society for Allergy and Clinical Immunology (Feb 2015)
Urticaria; NICE CKS, May 2016 (UK access only)
Zuberbier T, Aberer W, Asero R, et al; The EAACI/GA(2) LEN/EDF/WAO Guideline for the definition, classification, diagnosis, and management of urticaria: the 2013 revision and update. Allergy. 2014 Jul69(7):868-87. doi: 10.1111/all.12313. Epub 2014 Apr 30.
Urticaria; DermNet NZ
Urticaria and angio-oedema: an overview; Primary Care Dermatology Society
Omalizumab for previously treated chronic spontaneous urticaria; NICE Technology Appraisal Guidance, June 2015
Sharma M, Bennett C, Cohen SN, et al; H1-antihistamines for chronic spontaneous urticaria. Cochrane Database Syst Rev. 2014 Nov 14(11):CD006137. doi: 10.1002/14651858.CD006137.pub2.
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