What tests should be done for a patient with a worsening rash initially diagnosed as poison ivy?

Diagnostic Tests for Worsening Rash Initially Diagnosed as Poison Ivy

For a patient with a worsening rash initially diagnosed as poison ivy, a skin biopsy should be performed to rule out more serious conditions such as Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN) or other blistering disorders. 1

Initial Assessment

• Perform a detailed history focusing on:

  • Date of rash onset and progression pattern 1
  • Presence of systemic symptoms (fever, malaise) 1
  • Presence of mucosal involvement (eyes, mouth, genitalia) 1
  • Respiratory symptoms (cough, dyspnea) 1
  • All medications taken in the previous 2 months, including over-the-counter remedies 1
  • Previous history of drug allergies 1

• Complete physical examination should include:

  • Extent of rash and epidermal detachment (document percentage of body surface area affected) 1
  • Presence of target lesions, purpuric macules, blisters 1
  • Examination of all mucosal sites 1
  • Vital signs and oxygen saturation 1

Essential Diagnostic Tests

• Skin biopsy from lesional skin adjacent to a blister for routine histopathology 1

  • This is crucial to differentiate between allergic contact dermatitis and more serious conditions

• Second biopsy from periblister lesional skin for direct immunofluorescence to exclude immunobullous disorders 1

• Laboratory investigations:

  • Complete blood count with differential 1
  • C-reactive protein and erythrocyte sedimentation rate 1
  • Comprehensive metabolic panel (electrolytes, liver and kidney function) 1
  • Coagulation studies 1

• Swabs from lesional skin for bacterial culture 1

• Clinical photographs to document the type and extent of lesions 1

Additional Tests Based on Clinical Presentation

• For suspected allergic contact dermatitis:

  • Patch testing to identify potential allergens (if history suggests possible new contactant) 1
  • Consider testing for common allergens including nickel, neomycin, fragrance, preservatives, and rubber chemicals 1

• For suspected systemic allergic reaction:

  • Specific IgE testing (skin or blood) if allergic cause is suspected 1

• For severe or atypical presentations:

  • Chest X-ray if respiratory symptoms are present 1
  • Mycoplasma serology if mucositis is prominent 1

When to Consider Alternative Diagnoses

• If the rash is worsening despite appropriate treatment for poison ivy 2
• If there is extensive mucosal involvement 1
• If there is skin sloughing or epidermal detachment >30% BSA 1
• If there are atypical target lesions or purpuric macules 1
• If there are systemic symptoms disproportionate to typical poison ivy 1

Differential Diagnosis to Consider

• Stevens-Johnson syndrome/toxic epidermal necrolysis 1
• Erythema multiforme (can occur following poison ivy exposure) 3, 4
• Bullous pemphigoid or other autoimmune blistering disorders 1
• Acute generalized exanthematous pustulosis 1
• Staphylococcal scalded skin syndrome 1
• Drug eruption 1

Management Considerations While Awaiting Test Results

• For severe poison ivy covering >30% body surface area:

  • Oral prednisone 1-2 mg/kg/day (maximum 60 mg daily) for 5-7 days followed by a 2-3 week taper to prevent rebound dermatitis 2, 5
  • Short courses of systemic corticosteroids (less than 14 days) are associated with higher rates of return visits 5

• For suspected SJS/TEN:

  • Immediate discontinuation of any potential culprit drugs 1
  • Urgent dermatology consultation 1
  • Hospitalization if extensive epidermal detachment or mucosal involvement 1

Pitfalls to Avoid

• Failing to consider SJS/TEN in a worsening rash with mucosal involvement 1
• Using too short a course of systemic corticosteroids for severe poison ivy (less than 14 days), which can lead to symptom rebound 5
• Relying solely on clinical appearance without appropriate diagnostic testing for a worsening rash 1
• Overlooking allergic contact dermatitis to topical treatments being used to treat the initial rash 1
• Missing erythema multiforme as a rare complication of poison ivy exposure 3, 4