What lab tests are recommended for patients presenting with hives (urticaria)?

Laboratory Testing for Urticaria (Hives)

For most patients with acute urticaria or mild chronic urticaria responding to antihistamines, no laboratory tests are necessary. 1

Diagnostic Approach Based on Clinical Presentation

Acute Urticaria (< 6 weeks duration)

• No routine laboratory testing is recommended unless suggested by specific history 1
• Consider targeted testing only when history suggests:
  • IgE-mediated allergic reactions: Skin-prick testing or specific IgE blood tests for suspected allergens (foods, latex, etc.)
  • Drug reactions: Detailed medication history
  • Suspected infection: Targeted testing based on symptoms

Chronic Urticaria (≥ 6 weeks duration)

For patients with severe or antihistamine-resistant chronic urticaria, a limited screening panel is recommended:

1. Basic screening tests:

   • Complete blood count with differential (to detect eosinophilia or leukopenia)
   • Erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP)
   • Thyroid function tests and thyroid autoantibodies (especially if autoimmune etiology suspected)

2. Additional tests based on clinical suspicion:

   • Autologous serum skin test (ASST) to screen for autoimmune urticaria (in specialized centers)
   • Complement C4 level (if angioedema without wheals is present)
   • Skin biopsy (if urticarial vasculitis suspected - lesions lasting >24 hours with purpura)

Special Clinical Scenarios

Chronic Urticaria with Angioedema

• Serum C4 as initial screening test for hereditary/acquired C1 inhibitor deficiency
• If C4 is low, proceed with quantitative and functional C1 inhibitor assays
• C1q levels (reduced in acquired C1 inhibitor deficiency)

Physical Urticarias

• Provocation testing to confirm diagnosis and assess trigger thresholds
• Examples: ice cube test for cold urticaria, pressure testing for delayed pressure urticaria

Urticarial Vasculitis

• Skin biopsy is essential for diagnosis
• Full vasculitis screen including complement levels (C3, C4)
• Autoimmune panel if systemic involvement suspected

Common Pitfalls to Avoid

1. Overinvestigation: Extensive laboratory testing is rarely helpful and not cost-effective for most patients with urticaria

2. Underinvestigation: Missing potential systemic causes in patients with chronic, treatment-resistant urticaria

3. Misinterpretation: Positive autoantibody tests without clinical correlation may lead to unnecessary treatments

4. Delayed diagnosis: Failing to recognize urticarial vasculitis or hereditary angioedema which require specific management

Clinical Pearls

• The diagnostic algorithm should be guided by clinical presentation and response to initial therapy
• Consider thyroid autoimmunity testing as it's present in 14% of chronic urticaria patients versus 6% in the general population 1
• Chronic urticaria may have an autoimmune basis in approximately 40-45% of cases 2, 3
• For patients with chronic urticaria, consider testing for Helicobacter pylori if other causes are not identified 1
• Recognize that in most cases of chronic urticaria (up to 80%), a specific cause will not be identified despite testing 4

Remember that urticaria diagnosis is primarily clinical, and laboratory investigations should be tailored to the individual presentation rather than ordered as a standard panel for all patients.