What is the cause and treatment for a raised, red, blanching rash on both legs?

Raised, Red, Blanching Rash on Both Legs: Diagnosis and Management

Primary Assessment

A bilateral raised, red, blanching rash on the legs most likely represents an inflammatory dermatosis (eczematous dermatitis, contact dermatitis, or drug eruption) rather than an infectious process, since blanching indicates intact vasculature and excludes purpuric/petechial conditions. 1, 2

Critical Red Flags to Exclude First

Before proceeding with inflammatory causes, immediately assess for life-threatening conditions:

• Fever + headache + systemic symptoms: Start doxycycline 100 mg twice daily immediately without waiting for laboratory confirmation to cover Rocky Mountain Spotted Fever and ehrlichiosis (mortality 5-10% and 3% respectively) 3
• Skin sloughing, vesicles, or mucosal involvement with >30% body surface area: This requires emergency hospitalization to burn unit, IV methylprednisolone 1-2 mg/kg, and immediate dermatology consultation for severe drug eruption 3
• Thrombocytopenia or hyponatremia with rash: Initiate doxycycline empirically 3

Differential Diagnosis Based on Clinical Context

Drug-Induced Eruption (Most Common if on Medications)

If the patient is on anticancer agents, chemotherapy, or recently started new medications, treat as drug-induced eruption:

• Grade 1-2 (mild-moderate): Apply alcohol-free moisturizers with 5-10% urea twice daily, low-potency topical corticosteroids (hydrocortisone 2.5% or alclometasone 0.05%) twice daily, and start oral tetracyclines (doxycycline 100 mg twice daily or minocycline 100 mg once daily) for at least 6 weeks 4, 3
• Grade 3 (severe): Initiate systemic corticosteroids (prednisone 0.5-1 mg/kg for 7 days with weaning over 4-6 weeks), interrupt the offending drug until rash is grade 1, and escalate to high-potency topical steroids twice daily 4

Contact Dermatitis or Eczematous Dermatitis

For inflammatory dermatoses without drug exposure:

• Apply hydrocortisone 2.5% to affected areas 3-4 times daily 3
• Use alcohol-free moisturizers with urea 10% three times daily 4
• Avoid mechanical stress (prolonged walking, tight socks) and chemical irritants (solvents, disinfectants) during acute phase 4, 5
• Consider patch testing if allergic contact dermatitis is suspected 3

Fungal Infection (Tinea Corporis)

If the rash has fine scaling, is itchy, and the patient has recent sports participation or swimming:

• Oral itraconazole 100 mg once daily for 15 days (87% mycological cure rate) or terbinafine 250 mg once daily for 1 week 4
• Topical antifungals may be insufficient for bilateral leg involvement 4

Management Algorithm

1. Exclude life-threatening causes (fever, systemic symptoms, skin sloughing) → Start doxycycline or hospitalize 3
2. Review medication history → If drug-related, initiate topical steroids + oral tetracyclines + moisturizers 4, 3
3. Assess for fungal features (scaling, athletic exposure) → Oral antifungals 4
4. Default to inflammatory dermatosis → Topical corticosteroids + urea moisturizers + avoid irritants 4, 3

Common Pitfalls

• Misdiagnosing inflammatory "red legs" as cellulitis: Blanching rashes are not cellulitis; antibiotics are unnecessary and potentially harmful for chronic inflammatory conditions 1, 2
• Failing to stop offending medications: Drug eruptions will not resolve without discontinuing the causative agent 4
• Undertreating with low-potency steroids: Grade 2-3 drug eruptions require high-potency topical steroids or systemic corticosteroids 4, 3
• Missing tick-borne illness: Always ask about tick exposure in the past 2 weeks; erythema migrans appears 7-14 days post-bite and may be partially purpuric on lower extremities 5

Monitoring and Reassessment

• Reassess after 2 weeks: If no improvement with topical steroids and moisturizers, escalate to systemic therapy or refer to dermatology 4
• Mark lesion borders with ink: Observe for 24-48 hours to differentiate expanding infectious processes from static inflammatory conditions 5
• Obtain bacterial culture if infection suspected: Failure to respond to oral antibiotics, painful lesions, pustules on arms/legs/trunk, or yellow crusts warrant culture and 14 days of targeted antibiotics 4