Skin rash

A rash is any change in the skin — redness, spots, blisters, scaling, itching or swelling. The vast majority are primary skin problems: contact irritation or allergy, eczema, psoriasis, hives (urticaria), fungal or bacterial infection, viral rashes, or reactions to heat, plants or medicines. But the skin is also a window onto the body, and some rashes are the outward sign of something internal: autoimmune disease (the butterfly rash of lupus, or the intensely itchy blistering rash of coeliac disease), liver disease (with itching and jaundice), thyroid disease, or a drug reaction. The most important job when faced with a rash is to spot the small number of dangerous patterns: a rash with a high fever, a rash that does not fade when pressed (which can signal meningococcal infection), a rapidly spreading rash with facial or mouth swelling and breathing difficulty (a severe allergic reaction), or a widespread painful blistering rash — all of which need emergency care.

• Contact irritation or allergy — a reaction to something touching the skin.
• Eczema (atopic dermatitis) and psoriasis — common chronic skin conditions.
• Hives (urticaria) — raised, itchy welts, often allergic.
• Infections — fungal (ringworm), bacterial, or viral rashes.
• Drug reactions — a new medication is a common trigger.
• Autoimmune disease — lupus (butterfly facial rash), dermatitis herpetiformis (coeliac disease).
• Liver disease — itching, sometimes with jaundice.
• Dangerous patterns — a non-blanching rash with fever, or a rash with facial swelling and breathing difficulty — are emergencies.

Most rashes are diagnosed by looking at them, and need no blood tests. Testing is guided by the accompanying picture: if an internal or autoimmune cause is suspected, ANA (lupus), inflammatory markers (CRP, ESR), a full blood count (infection, eosinophils in allergy), liver function tests (itch with liver disease), and coeliac serology (for the blistering rash of dermatitis herpetiformis) may be relevant. An allergic cause may warrant allergy testing separately. Mediora.AI can surface the autoimmune, liver and inflammatory pattern from your results when a rash suggests an internal cause; the rash itself is best assessed by seeing it — a GP or dermatologist — and a dangerous rash needs emergency care, not tests.