What is the appropriate work‑up for a persistent pruritic macular scaly rash?

Work-Up for Non-Resolving Pruritic Macular Scaly Rash

Begin with potassium hydroxide (KOH) preparation to rule out dermatophyte infection, as scaly pruritic rashes with alopecia or plaque formation are characteristic of tinea corporis or tinea capitis, which require specific antifungal therapy. 1

Initial Diagnostic Steps

Immediate Bedside Testing

• Perform 10% KOH preparation of skin scrapings from the active border of scaly lesions to identify fungal hyphae, as this is the most rapid diagnostic test for tinea infections 1
• Examine for distribution patterns: Look specifically for involvement of groin, feet, interdigital spaces, and scalp, as these suggest dermatophyte infection 1
• Assess for secondary bacterial infection: Look for honey-crusted lesions, pustules, or purulent discharge that would indicate superimposed bacterial infection requiring culture 1

Key Historical Elements

• Travel history and environmental exposures: Contact sports participation, communal shower use, or sharing of towels/equipment increases risk of tinea corporis 1, 2
• Personal or family history of atopy: This suggests atopic dermatitis rather than fungal infection 2
• Medication history: Recent initiation of biologics or immunosuppressants can cause drug-induced rashes 1
• HIV risk factors: Sexual history and intravenous drug use, as HIV-associated pruritus can present with minimal skin findings 1

Laboratory Work-Up Based on Clinical Presentation

If KOH is Positive or Fungal Infection Suspected

• Fungal culture if KOH is equivocal or treatment fails, to confirm species and guide therapy 1
• No additional laboratory testing needed if classic tinea presentation with positive KOH 1

If KOH is Negative - Proceed with Systemic Work-Up

• Complete blood count with differential: To assess for eosinophilia (suggests atopic dermatitis, drug reaction, or HIV-associated pruritus) 1
• Comprehensive metabolic panel: To evaluate for hepatic or renal causes of pruritus 1
• HIV testing: Especially if risk factors present, as pruritus may be presenting feature 1
• Hepatitis panel (A, B, C, E): Cholestatic pruritus from viral hepatitis presents with minimal rash 1
• Thyroid function tests: Hypothyroidism can cause pruritic dry skin 1

If Diagnosis Remains Unclear After Initial Work-Up

• Skin biopsy with histopathology: Essential if considering psoriasis, atopic dermatitis, or cutaneous T-cell lymphoma 1, 2
• Viral PCR of skin lesion: If vesicular component or concern for herpes simplex/varicella zoster 1
• Patch testing: If contact dermatitis suspected from occupational or environmental exposures 2

Critical Diagnostic Pitfalls to Avoid

• Do not assume psychogenic pruritus without completing full systemic work-up, as one-third of chronic pruritus patients have underlying medical conditions 1
• Do not miss scabies, which presents with severe pruritus and minimal skin signs, particularly in immunocompromised or HIV-positive patients 1
• Avoid treating empirically with systemic corticosteroids before establishing diagnosis, as this can worsen fungal or bacterial infections and mask underlying malignancy 1, 3
• Rule out drug-induced rash before escalating immunosuppression, particularly if patient recently started new medications 1

Morphologic Clues to Guide Differential

Scaly Plaques with Alopecia

• Tinea capitis or tinea corporis - requires KOH and fungal culture 1

Scaly Itchy Eruption in Groin and Feet

• Tinea cruris and tinea pedis - KOH preparation diagnostic 1

Macular Erythema with Pruritus and Desquamation

• Drug-induced rash from biologics or chemotherapy - requires medication review and possible drug discontinuation 1

Scaly Plaques on Extensor Surfaces

• Psoriasis - skin biopsy shows characteristic histopathology 1, 4

Flexural Involvement with Lichenification

• Atopic dermatitis - personal/family history of atopy, elevated IgE 5, 6

When to Refer to Dermatology

• Immediate referral: If diagnostic uncertainty after initial work-up, concern for malignancy, or failure of primary care management 3, 7
• Urgent referral: If rash covers >30% body surface area, systemic symptoms present, or suspected severe drug reaction 1
• Routine referral: If requiring prolonged systemic therapy or consideration of phototherapy 3, 8