What laboratory tests should be drawn for a patient presenting with idiopathic urticaria?

Laboratory Tests for Idiopathic Urticaria

For patients with idiopathic urticaria, limited laboratory testing is recommended, with a full blood count and erythrocyte sedimentation rate being the most important initial tests for severe cases not responding to antihistamines. 1

Initial Laboratory Evaluation for Chronic Urticaria

• No routine laboratory tests are required for the majority of patients with mild chronic urticaria who respond to H1 antihistamines 1
• For patients with more severe disease not responding to antihistamines, a basic screening profile should include:
  • Complete blood count with differential - to detect eosinophilia (suggesting parasitic infections) or leukopenia (suggesting systemic lupus erythematosus) 1
  • Erythrocyte sedimentation rate (ESR) - usually normal in chronic ordinary urticaria but may be elevated in urticarial vasculitis and autoinflammatory syndromes 1
  • C-reactive protein (CRP) - to assess for systemic inflammation 1
  • Thyroid autoantibodies and thyroid function tests - especially if autoimmune etiology is suspected, as thyroid autoimmunity is more prevalent in chronic urticaria (14%) than in the general population (6%) 1
  • Total IgE level and IgG anti-thyroid peroxidase (anti-TPO) antibodies - to help identify autoimmune or autoallergic urticaria 1

Additional Testing Based on Clinical Presentation

• For suspected autoimmune urticaria:

  • Autologous serum skin test (ASST) - a reasonably sensitive and specific screening test for functional autoantibodies 1
  • Basophil histamine release assay - gold standard for detecting functional autoantibodies (where available) 1
  • CU index - to determine presence of antibodies against IgE, FcεRI, or anti-FcεRII 1

• For angioedema without wheals:

  • Serum C4 - initial screening test for hereditary and acquired C1 inhibitor deficiency 1
  • If C4 is low, quantitative and functional C1 inhibitor assays 1
  • C1q levels - reduced in acquired C1 inhibitor deficiency 1

• For suspected urticarial vasculitis:

  • Lesional skin biopsy - essential to confirm small-vessel vasculitis 1, 2
  • Serum complement assays (C3 and C4) - to distinguish normocomplementemic from hypocomplementemic disease 1, 2
  • Full vasculitis screen 1

Special Considerations

• For suspected autoinflammatory disease: Test for elevated inflammatory markers (CRP and ESR), paraproteinemia, and consider gene mutation analysis for hereditary periodic fever syndromes 1

• For suspected physical urticarias: Standardized provocation testing according to international consensus recommendations 1

• For monitoring disease activity: Use the 7-Day Urticaria Activity Score to assess disease severity and treatment response 1

Clinical Pearls and Pitfalls

• Laboratory investigations should be guided by the patient's history and clinical presentation - not performed indiscriminately 1

• A positive baseline autologous serum skin test in acute urticaria is a significant predictor for progression to chronic urticaria (odds ratio 5.91) 3

• Patients with autoimmune chronic urticaria are more likely to have low total IgE levels and elevated IgG anti-TPO levels 1

• The diagnostic workup of chronic spontaneous urticaria should follow the "7 Cs" approach: confirm diagnosis, look for causes, identify conditions that modify disease activity, check for comorbidities, assess consequences, evaluate predictors of disease course, and monitor disease activity 1

• Basopenia and presence of anti-thyroid peroxidase antibodies are associated with positive autologous serum skin test results and may predict progression from acute to chronic urticaria 3