What investigations should be done in a patient presenting with urticaria?

Investigations in Urticaria

For acute urticaria, no routine laboratory testing is required unless the patient's history suggests a specific allergic trigger that needs confirmation. 1

Acute Urticaria (< 6 weeks duration)

• No routine testing is recommended unless history points to a specific allergic cause requiring confirmation 1, 2
• The diagnosis is entirely clinical, based on characteristic intensely pruritic wheals lasting 2-24 hours 2, 3
• If a specific trigger (food, drug, insect sting) is suspected and identification would change management, consider specific IgE testing (skin or blood) 4

Chronic Spontaneous Urticaria (≥ 6 weeks duration)

Basic laboratory testing only—the "7 Cs" approach includes:

• Complete blood count with differential 1, 2
• C-reactive protein or ESR 1, 2
• Total IgE level 1, 2
• IgG anti-thyroid peroxidase (anti-TPO) 1, 2

Rationale for These Specific Tests

• Low total IgE with elevated anti-TPO suggests autoimmune CSU 2
• High anti-TPO/total IgE ratio is the best surrogate marker for autoimmune CSU, which predicts more severe disease and poorer response to antihistamines 1, 2
• Patients with autoimmune CSU are more likely to have very low total IgE levels and elevated IgG-anti-TPO 1

Additional Testing Only If Red Flags Present

Do NOT perform extensive laboratory workups routinely. 1, 2 Only pursue targeted testing if specific clinical features suggest underlying disease:

• If wheals last > 24 hours or leave bruising/hyperpigmentation: Skin biopsy to evaluate for urticarial vasculitis 1, 2
• If recurrent fever, joint/bone pain, or malaise: Check inflammatory markers (CRP, ESR), consider paraproteinemia testing in adults, and evaluate for autoinflammatory syndromes 1, 4
• If angioedema without wheals: Test complement C4, C1-inhibitor levels and function; add C1q and C1-inhibitor antibodies if acquired angioedema suspected 1, 2
• If patient requires regular steroid use for control: Consider referral to allergist-immunologist for evaluation of systemic disease 1

Chronic Inducible Urticaria (Physical Urticarias)

• Diagnosis confirmed by standardized provocation testing according to international consensus recommendations 1, 4
• Ask the patient: "Can you make your wheals appear? Can you bring out your wheals?" to identify physical triggers 1, 4
• No laboratory testing needed beyond provocation testing and trigger threshold assessment 1
• Physical urticarias include dermographism, cold, cholinergic (heat), delayed pressure, solar, and exercise-induced 2, 5

Disease Monitoring Tools (Not Diagnostic Tests)

• Urticaria Control Test (UCT): Score < 12 indicates poor control requiring treatment escalation 2
• Angioedema Control Test (AECT): Score < 10 indicates poor control 2
• 7-Day Urticaria Activity Score (UAS7): Maximum 42 points, used to quantify disease activity 1

Common Pitfalls to Avoid

• Do not order extensive allergy panels, food-specific IgE panels, or comprehensive autoimmune workups in uncomplicated chronic spontaneous urticaria 1, 2
• Do not routinely perform imaging studies unless specific clinical features suggest underlying systemic disease 4
• Do not confuse chronic spontaneous urticaria with chronic inducible urticaria—the former has no identifiable physical trigger, while the latter is reproducibly induced by specific stimuli 2
• Remember that individual wheal duration is the most critical diagnostic feature—wheals lasting > 24 hours suggest urticarial vasculitis, not typical urticaria 1, 2