I have a mildly pruritic erythematous rash on my forearm without signs of infection; how should I evaluate and treat it?

Evaluation and Management of Mildly Pruritic Erythematous Forearm Rash Without Infection

Begin with liberal application of emollients at least twice daily and a non-sedating antihistamine (fexofenadine 180 mg or loratadine 10 mg daily) while you systematically exclude life-threatening causes and treatable systemic diseases. 1

Immediate Red-Flag Assessment (Rule Out Life-Threatening Causes First)

Before attributing your rash to a benign dermatosis, you must exclude Rocky Mountain spotted fever (RMSF) and other tickborne rickettsial diseases, which carry a 5–10% case-fatality rate and kill half their victims within 9 days of symptom onset. 2, 3

Critical Questions to Ask Yourself Right Now

• Do you have fever, headache, or severe myalgias? If yes, start doxycycline 100 mg twice daily immediately without waiting for laboratory confirmation, because fewer than 50% of RMSF patients develop a rash in the first 3 days and up to 20% never develop one at all. 2, 3
• Have you had any tick exposure or been in wooded/grassy areas in the past 2 weeks? RMSF typically begins on ankles, wrists, or forearms 2–4 days after fever onset, and up to 40% of patients do not recall a tick bite. 2, 3
• Is the rash spreading to your palms, soles, or developing central petechiae? This progression is characteristic of RMSF and demands immediate empiric doxycycline. 2, 3

If you have fever + rash + headache + any tick/outdoor exposure, go to urgent care or the emergency department today for empiric doxycycline. 2, 3

Symptomatic Management While Investigating (Start Today)

While you work through the diagnostic algorithm below, begin these universal first-line therapies immediately:

• Apply emollients liberally at least twice daily to all affected areas, because xerosis (dry skin) lowers the threshold for pruritus and is present in nearly all chronic itch conditions. 1
• Take a non-sedating antihistamine daily (fexofenadine 180 mg or loratadine 10 mg) for symptomatic itch relief. 1
• Avoid hot water, keep nails short, and use stress-reduction techniques to break the itch-scratch cycle. 1

Systematic Diagnostic Workup (If No Red Flags Present)

Step 1: Detailed Exposure and Medication History

• Recent medication changes in the past 2–8 weeks? Drug-induced pruritus accounts for approximately 12.5% of all drug reactions and can present with or without a rash. 4, 1
• New soaps, detergents, lotions, or occupational exposures? Contact dermatitis is a leading cause of localized forearm rash due to direct skin contact with allergens or irritants. 3, 5
• Recent viral upper respiratory infection? Viral exanthems are the most common cause of maculopapular rashes, though they typically involve the trunk and extremities more diffusely. 2

Step 2: Characterize the Rash Morphology and Distribution

• Well-demarcated erythematous plaques with silvery scale on extensor surfaces? This suggests psoriasis, which is typically symmetric and involves elbows, knees, and scalp. 3
• Poorly demarcated erythema in flexural areas with a personal/family history of atopy? This points toward atopic dermatitis (eczema), which predominantly affects flexural surfaces. 3
• Persistent erythematous patches that do NOT respond to topical corticosteroids after 2–4 weeks? This is a red flag for cutaneous T-cell lymphoma and requires dermatology referral for skin biopsy. 3

Step 3: Laboratory Testing (If Rash Persists Beyond 2 Weeks or Systemic Symptoms Develop)

If your rash does not improve with emollients and antihistamines after 2 weeks, or if you develop systemic symptoms (fatigue, weight loss, night sweats), order the following panel:

• Complete blood count with differential to screen for polycythemia vera, lymphoma, and anemia. 1
• Comprehensive iron studies (serum iron, ferritin, transferrin saturation) because iron deficiency causes generalized pruritus in approximately 25% of cases and resolves completely within days of iron replacement. 1
• Liver function tests, total bilirubin, and serum bile acids to identify cholestatic liver disease, which often presents with pruritus affecting palms and soles that worsens at night. 1
• Renal panel (urea, creatinine, electrolytes) to detect uremic pruritus from chronic kidney disease. 1

Cause-Specific Treatment Algorithms

If Iron Deficiency Is Identified

• Oral iron supplementation (ferrous sulfate 325 mg daily or equivalent elemental iron 65 mg daily) leads to complete cessation of pruritus within days to weeks. 1

If Contact Dermatitis Is Suspected

• Identify and eliminate the offending agent (new soap, lotion, occupational exposure). 5
• Apply a mid-potency topical corticosteroid (e.g., triamcinolone 0.1% cream) twice daily for 1–2 weeks to the affected area. 1

If Atopic Dermatitis (Eczema) Is Diagnosed

• Continue liberal emollients at least twice daily as the cornerstone of therapy. 1
• Apply a low- to mid-potency topical corticosteroid (e.g., hydrocortisone 2.5% or triamcinolone 0.1%) to inflamed areas twice daily for 1–2 weeks. 1

If Psoriasis Is Suspected

• Refer to dermatology for confirmation and consideration of topical vitamin D analogs, topical corticosteroids, or phototherapy. 3

If Neuropathic Pruritus Is Suspected (Burning, Stinging Quality)

• Gabapentin 300 mg at bedtime, titrated to 300–1200 mg daily in divided doses over 1–2 weeks based on response and tolerability. 1

When to Refer or Escalate Care

• Refer to dermatology if the rash persists beyond 2–4 weeks despite first-line therapy or if the diagnosis remains uncertain. 1
• Refer to dermatology immediately if the rash does not respond to topical corticosteroids after 2–4 weeks, as this raises concern for cutaneous T-cell lymphoma. 3
• Go to urgent care or the emergency department today if you develop fever, severe headache, or systemic symptoms (hypotension, tachycardia, altered mental status), as these may indicate RMSF or another life-threatening condition. 2, 3

Key Clinical Pitfalls to Avoid

• Do not assume a localized forearm rash is benign without asking about fever, headache, and tick exposure, because RMSF can begin on the wrists/forearms and up to 40% of patients do not recall a tick bite. 2, 3
• Do not rely on antihistamines alone for systemic causes of pruritus (iron deficiency, cholestatic liver disease, uremia), as they are generally ineffective for these conditions. 1
• Do not delay a comprehensive medication review, because drug-induced pruritus is common (12.5% of drug reactions) and often reversible with discontinuation of the offending agent. 4, 1
• Do not attribute persistent, non-responsive rash to "eczema" without considering cutaneous T-cell lymphoma, which requires skin biopsy for diagnosis. 3