How do you differentiate a vasculitis rash from a viral rash?

Differentiating Vasculitis Rash from Viral Rash

The key distinguishing feature is that vasculitis rash presents as palpable purpura that persists beyond 24 hours and does not blanch with pressure, while viral rashes typically present as blanching maculopapular eruptions that evolve more rapidly and resolve within days. 1, 2, 3

Critical Clinical Features to Assess

Lesion Characteristics

Vasculitis rash:

• Palpable purpura is the hallmark—raised, non-blanching lesions that you can feel with your fingertips 3, 4, 5
• Individual lesions persist >24 hours (often days to weeks) 6
• Non-blanching quality when pressure is applied (use a glass slide or clear plastic to compress the lesion) 2, 4
• Lesions may progress to ulceration, necrosis, or bullae in severe cases 3, 7
• Distribution typically favors dependent areas (lower extremities) due to hydrostatic pressure 3, 4

Viral rash:

• Blanching maculopapular eruptions that are flat or minimally raised 8, 1
• Individual lesions last 2-24 hours and evolve in successive crops 8, 1
• More generalized distribution, often trunk-predominant 8, 1
• Petechiae may occur with enteroviruses but progress more slowly than bacterial/vasculitic causes 1
• Lesions do not typically ulcerate or become necrotic 8

Temporal Evolution

Vasculitis:

• Lesions develop over days and persist for extended periods 6, 3
• New lesions may continue to appear while old lesions remain visible 3, 5
• Healing leaves post-inflammatory hyperpigmentation or scarring 3, 4

Viral:

• Rapid onset with fever and viral prodrome (2-4 days) 8, 1
• Lesions appear in successive waves over hours to days 8, 1
• Complete resolution typically within 1-2 weeks without scarring 8, 1

Associated Clinical Context

Systemic Features Suggesting Vasculitis

• Multiorgan involvement: renal dysfunction (hematuria, proteinuria), pulmonary symptoms, neurologic deficits, or gastrointestinal bleeding 8, 3
• Constitutional symptoms that persist beyond typical viral illness duration (>7-10 days) 3, 4
• Arthralgias or arthritis, particularly in large joints 3, 4
• History of autoimmune disease, hepatitis B/C, or recent medication exposure 3, 4

Features Suggesting Viral Etiology

• Acute febrile illness with typical viral prodrome (myalgias, headache, upper respiratory symptoms) 8, 1
• Exposure history or epidemic context (daycare, school outbreaks) 1
• Associated symptoms: conjunctivitis, pharyngitis, lymphadenopathy 8
• Clinical improvement within 5-7 days 8, 1

Diagnostic Confirmation

Essential Investigations for Suspected Vasculitis

Skin biopsy is the gold standard and should be performed on the most tender, reddish, or purpuric lesion extending to the subcutis 8, 3

• Histopathology shows fibrinoid necrosis, leukocytoclasia, endothelial damage, and red cell extravasation 8, 6, 3
• Direct immunofluorescence distinguishes IgA-associated vasculitis (Henoch-Schönlein purpura) from IgG/IgM-associated forms, which has prognostic significance 3, 5

Laboratory workup for vasculitis:

• ANCA testing (indirect immunofluorescence and ELISA for PR3 and MPO) in appropriate clinical context 8
• Complete blood count, comprehensive metabolic panel, urinalysis with microscopy 8, 4
• Inflammatory markers (ESR, CRP) 8, 4
• Complement levels (C3, C4) to distinguish normocomplementemic from hypocomplementemic disease 6, 4
• Hepatitis B and C serologies 8, 3

Viral Rash Confirmation

• Clinical diagnosis is usually sufficient when presentation is typical 8, 1
• Viral PCR or serology only if diagnosis unclear or public health concern 8
• Normal or mildly elevated inflammatory markers 1

Critical Pitfalls to Avoid

Never dismiss a non-blanching petechial rash without thorough evaluation—meningococcemia can present similarly to viral illness initially but requires urgent treatment 1, 2

Do not rely solely on rash distribution: While vasculitis favors lower extremities, serious infections like Rocky Mountain Spotted Fever and meningococcemia can also cause petechiae on extremities, including palms and soles 2

Absence of systemic symptoms does not exclude vasculitis—cutaneous small-vessel vasculitis can be isolated to skin in 30-50% of cases 3, 4

In darker-skinned patients, petechial and purpuric rashes may be difficult to recognize, increasing risk of delayed diagnosis—palpate all suspicious lesions 2

Urticarial vasculitis is a specific entity where lesions resemble urticaria but persist >24 hours; this requires biopsy for diagnosis as it behaves differently from both typical urticaria and typical vasculitis 6