What laboratory tests are recommended for patients presenting with urticaria?

Laboratory Testing for Urticaria

For most patients with urticaria, limited laboratory testing is recommended, focusing on differential blood count, C-reactive protein/ESR, total IgE level, and IgG anti-thyroid peroxidase (anti-TPO) levels for chronic spontaneous urticaria cases. 1

Acute vs. Chronic Urticaria Testing Approach

Acute Urticaria (≤6 weeks)

• No routine testing is recommended unless the patient's history suggests a specific underlying cause 2, 1
• If history suggests allergic reaction:
  • Consider specific testing to confirm allergen trigger
  • Document potential triggers from history (foods, drugs, insect stings)

Chronic Spontaneous Urticaria (>6 weeks)

• Basic recommended tests 2, 1:

  • Differential blood count (to detect eosinophilia or leukopenia)
  • C-reactive protein level and/or ESR
  • Total IgE level (low levels suggest autoimmune CSU)
  • IgG anti-thyroid peroxidase (anti-TPO) levels

• Additional testing for non-responders to antihistamines 2, 1:

  • CU index to detect antibodies against IgE, FcεRI, or anti-FcεRII
  • Autologous serum skin test (ASST) as a screening test for autoimmune urticaria 2
  • Thyroid function tests (especially if autoimmune etiology is suspected)

Chronic Inducible Urticaria

• Standardized provocation testing to confirm diagnosis and assess trigger threshold 2, 1
• Testing protocols should be specific to the suspected physical trigger (cold, heat, pressure, etc.)

Special Cases Requiring Specific Testing

Angioedema Without Wheals

• Serum C4 level as initial screening test for hereditary and acquired C1 inhibitor deficiency 2, 1
• If C4 is low:
  • Quantitative and functional C1 inhibitor assays
  • C1q levels (reduced in acquired C1 inhibitor deficiency)
  • Consider genetic testing if hereditary angioedema is suspected 2

Urticarial Vasculitis

• Lesional skin biopsy is essential to confirm small-vessel vasculitis 2, 1
• Complete vasculitis screen including:
  • Serum complement assays (C3 and C4)
  • Antinuclear antibody titer
  • ESR

Monitoring Disease Activity and Treatment Response

• Use validated assessment tools 2, 1:
  • Urticaria Control Test (UCT) for patients with wheals
  • Angioedema Control Test (AECT) for patients with angioedema
  • 7-Day Urticaria Activity Score to quantify disease activity

Common Pitfalls to Avoid

• Avoid excessive laboratory testing in acute urticaria without specific indications 2, 1
• Don't rely on food skin tests for chronic urticaria diagnosis as they have poor reliability 3
• Avoid missing urticarial vasculitis - consider biopsy when lesions last >24 hours, are painful, or leave residual bruising/pigmentation
• Remember that 80-90% of chronic urticaria cases are idiopathic despite extensive testing 4, 5
• Don't overlook autoimmune thyroiditis as a common comorbidity (approximately 20% of chronic spontaneous urticaria cases) 5

By following this structured approach to laboratory testing in urticaria, clinicians can avoid unnecessary investigations while ensuring appropriate diagnosis and management for optimal patient outcomes.