Rash
An undifferentiated rash is approached in primary care by risk-stratifying first: deciding whether to observe, treat empirically, test, or refer based on how likely a serious cause is, since the differential spans infectious, allergic, drug-related, and autoimmune causes.
Symptoms
Presentation varies enormously by cause, but the pattern (distribution, timing, and associated symptoms like fever or pain) is usually more useful than the appearance of the rash alone.
Causes and risk
New medications, recent infections, and known allergies are the most useful clues. Drug reactions typically appear days to a few weeks after starting a new medication; a reaction 2-6 weeks after starting a drug raises concern for drug reaction with eosinophilia and systemic symptoms (DRESS), a more serious drug hypersensitivity syndrome.
How it is evaluated
A focused history covering timing, new medications, fever, and mucosal involvement (mouth, eyes, genitals) narrows the differential far more efficiently than the rash's appearance alone.
Treatment
Management depends entirely on the underlying cause once identified, ranging from no treatment needed to antihistamines, topical care, or stopping an offending medication.
When to seek care
Non-blanching purpura with fever, skin pain with blistering or peeling, mucosal involvement (mouth, eye, genital sores), facial swelling with fever after starting a new drug, or any airway or facial swelling are emergencies -- possible meningococcemia, Stevens-Johnson syndrome/TEN, DRESS syndrome, or anaphylaxis -- and need a 911 call or immediate emergency care, not a wait-and-see approach.