How do I differentiate between a macular rash and a raised urticarial rash?

Differentiating Macular Rash from Raised Urticarial Rash

The key distinction is simple: urticarial lesions (wheals) are raised, palpable, and blanch with pressure, while macular rashes are flat, non-palpable, and may or may not blanch depending on their etiology. 1

Primary Distinguishing Features

Tactile Assessment (Most Important)

• Run your finger across the lesion: Urticarial wheals create a palpable elevation above the skin surface, while macular lesions remain completely flat 1, 2
• Blanching test: Apply firm pressure with a glass slide or your finger—urticarial lesions blanch completely and temporarily disappear, then rapidly return when pressure is released 2
• Macular lesions may blanch (if erythematous) or may not blanch (if petechial/purpuric) 1, 2

Temporal Characteristics

• Individual urticarial wheals last 2-24 hours and then completely resolve without residual marks, moving to different locations 1
• If lesions persist in the same location for >24 hours and resolve with hyperpigmentation or bruising, consider urticarial vasculitis rather than ordinary urticaria 1, 3
• Macular rashes typically persist longer in the same location and may evolve through color changes (pink to red to brown) 1

Visual Characteristics

• Urticarial wheals: Pink to red, edematous, well-demarcated borders, often with central pallor, may be annular or polycyclic 1, 4
• Macular rashes: Flat discolorations without elevation, may be pink, red, or violaceous, borders vary by etiology 1, 2

Critical Pitfalls to Avoid

Target-Like Lesions Are Not Typical Urticaria

• If you see target-like or iris lesions with both raised and flat components, this suggests urticarial vasculitis, erythema multiforme, or urticaria multiforme—not simple urticaria 3, 4
• Urticaria multiforme in children presents with blanchable, annular wheals with dusky ecchymotic centers that can mimic erythema multiforme, but the lesions are transient (<24 hours) and fully blanchable 4

Fever Changes the Differential

• Fever is uncommon in ordinary urticaria 3, 5
• Urticaria with fever suggests autoinflammatory syndromes (Muckle-Wells syndrome, Schnitzler syndrome), urticarial vasculitis, or viral exanthem 1, 3, 5
• These conditions require elevated inflammatory markers (ESR, CRP) and may need skin biopsy for definitive diagnosis 1, 3

Distribution Patterns Provide Clues

• Palms and soles involvement: More common with macular rashes from Rocky Mountain spotted fever, secondary syphilis, or viral exanthems than with ordinary urticaria 1
• Urticaria typically spares the face, while many macular viral exanthems (measles, rubella) prominently involve the face 1

Practical Bedside Algorithm

1. Palpate the lesion: Raised = urticarial; Flat = macular 1, 2
2. Apply pressure: Complete blanching with rapid return = urticaria; Persistent discoloration = consider vasculitis or purpura 1, 2
3. Check timing: Individual lesions <24 hours = urticaria; >24 hours in same location = not ordinary urticaria 1, 3
4. Assess for fever: Present = consider alternatives to simple urticaria 3, 5
5. Look for residual changes: Urticaria resolves without trace; hyperpigmentation or bruising suggests vasculitis 1, 3

When Diagnostic Uncertainty Persists

• Photograph the lesions for documentation and to track evolution over time 1
• If lesions persist >24 hours in the same location despite antihistamines, perform lesional skin biopsy to evaluate for urticarial vasculitis 1
• Check inflammatory markers (ESR, CRP) if fever or systemic symptoms are present 1, 3
• Consider autoinflammatory syndromes if recurrent episodes of urticaria-like lesions occur with fever and poor antihistamine response 3, 5