Common Rashes: What to Look For

Rashes can arise from inflammation, infection, medicines, allergy, bleeding into the skin or other processes. No single colour, shape or glass test result identifies the cause by itself.

A rash is a description, not a diagnosis A rash is a visible or palpable change affecting an area of skin. A skin lesion is one altered area within that rash or elsewhere on the skin. Clinicians describe what they can see and feel before naming a possible condition. This approach reduces errors caused by matching one photograph to one diagnosis. Morphology means the form and surface of a lesion. Distribution means where lesions appear and how they are arranged across the body. Clinicians describe the lesion itself A macule is a small flat area of colour change. A larger flat area is often called a patch. A papule is a small solid raised lesion. A plaque is a broader raised or thickened area with a relatively flat surface. A vesicle is a small fluid filled blister. A pustule contains pus, while a large fluid filled blister is described simply as a large blister in this lesson. Scale means flakes of outer skin. Crust is dried fluid, blood or pus on the surface. A wheal is a temporary raised swelling, often seen with urticaria or hives. These descriptions can overlap and do not diagnose the cause. Distribution and change over time provide clues Clinicians note whether a rash is localised, widespread, symmetrical, one sided or limited to an area of contact. They also check skin folds, palms, soles, scalp and nails when relevant. A rash beneath a watch strap can support contact dermatitis. It does not prove allergy because irritation, friction, moisture or infection can produce a similar pattern. A symmetrical rash does not automatically mean an internal illness. Eczema, scabies, medicine reactions and other conditions can also appear symmetrically. Timing matters. Clinicians ask whether lesions appeared within minutes, developed over days, move between sites or remain fixed. They ask about new medicines, recent illness, travel, bites, close contacts, work exposures and skin products. A trigger provides context rather than confirmation. Skin changes can look different across skin tones Inflamed skin may look pink or red on lighter skin. On brown or black skin, it may look purple, grey, dark brown or simply darker than nearby skin. Some colour changes can be difficult to see. Warmth, swelling, texture and tenderness may provide additional information. Good lighting and comparison with nearby skin can help. Clinical resources should include several skin tones rather than one reference image. Itch, pain and associated symptoms matter Itch can occur with eczema, contact dermatitis, urticaria, scabies, fungal infection and several other conditions. Itch alone cannot identify an allergy. Pain, tenderness or burning changes the differential diagnosis. Severe pain that seems excessive for the visible skin change needs emergency assessment. Fever, confusion, breathing difficulty, joint pain, swelling or feeling very unwell can change urgency. So can lesions affecting the mouth, eyes or genitals. The glass test assesses blanching only Blanching means colour fades when firm pressure temporarily moves blood from small vessels near the skin surface. The colour returns when pressure is released. A non blanching rash remains visible under pressure. Petechiae are small non blanching spots, while purpura describes larger non blanching spots or patches. Petechiae and purpura can occur when blood leaves small vessels or when vessels and clotting are affected. Causes range from minor pressure effects to serious infection or blood disorders. The glass test does not diagnose meningitis or explain why a rash is non blanching. It is also harder to interpret on some skin tones. Check paler areas such as the palms, soles, inside the eyelids or mouth when appropriate. Do not delay emergency help while repeatedly performing the test. Meningococcal disease may occur without a rash A rapidly spreading non blanching rash with fever or severe illness raises concern about meningococcal disease. This pattern requires emergency assessment. However, meningococcal disease can occur without any rash. An early rash may also blanch before becoming non blanching. Severe headache, stiff neck, confusion, drowsiness, seizure, breathing difficulty or mottled skin also require urgent action. Do not wait for every feature to appear. A person who feels well can still develop a non blanching rash from another cause. They need immediate clinical advice rather than a home diagnosis. Common patterns provide clues, not answers Dry, itchy and scaly skin can occur with eczema or contact dermatitis. A local exposure pattern can support contact dermatitis but does not identify the exact trigger. Urticaria usually produces raised itchy wheals that can change shape or move between areas. Infection, medicines, heat, pressure and other triggers can cause it without a true allergy. An itchy ring shaped rash can occur with ringworm, which is a fungal infection. Several non fungal conditions can also form rings. Scabies often causes intense itch that is worse at night, sometimes affecting close contacts. The rash can be difficult to recognise without examining the usual body sites. Painful, hot and swollen skin can occur with cellulitis. Rapid spread, fever or worsening illness increases urgency. A painful one sided group of blisters can occur with shingles. A facial rash near the eye, eye pain or vision change needs urgent assessment. Medicine related rashes need careful assessment A new medicine can cause a mild rash, urticaria, anaphylaxis or a delayed severe skin reaction. Timing and associated symptoms help distinguish these possibilities. Anaphylaxis can involve throat or tongue swelling, breathing difficulty, faintness or confusion. A rash may be present, but anaphylaxis can occur without obvious skin changes. Stevens Johnson syndrome is a rare medical emergency. Warning features include a painful spreading rash, blisters, peeling skin and sores affecting mucosal surfaces such as the mouth, eyes or genitals. Seek prompt professional advice for a new rash after starting a medicine. Call 999 or go to A&E for severe features and bring the medicines with you. Do not restart a medicine suspected of causing a severe reaction unless a specialist advises this. Do not stop other regular medicines without clinical advice. Examination and tests answer selected questions Many rashes are assessed through symptoms, background and examination. Photographs can show how a temporary or changing rash developed, but they do not replace assessment. A swab can test material from a weeping, crusted or blistered lesion when infection is suspected. A skin scraping can support testing for fungus or scabies. Blood tests may assess infection, inflammation, platelets, clotting or organ involvement. No routine blood panel identifies every rash cause. Patch testing can investigate selected allergic contact dermatitis. It is different from testing for immediate food or medicine allergy. A skin biopsy removes a small tissue sample for laboratory examination. It can distinguish selected inflammatory, blistering or cancerous processes but is not needed for most rashes. When to get help Call 999 or go to A&E for a non blanching rash with fever, rapid spread, severe headache, stiff neck, confusion, drowsiness or breathing difficulty. Do not drive yourself. For a new non blanching rash without severe illness, contact NHS 111 immediately. For a child, follow current NHS advice and seek emergency assessment. Call 999 for sudden throat or tongue swelling, breathing difficulty, fainting, blue or grey colour, confusion or reduced responsiveness. These can indicate anaphylaxis. Call 999 or go to A&E for a painful spreading rash with blisters, peeling skin or sores in the mouth, eyes or genitals. This can indicate a severe medicine reaction. Call 999 or go to A&E for skin pain far greater than expected, black, purple or grey patches, rapid swelling or severe illness. These can indicate a deep tissue infection. Request urgent GP or NHS 111 assessment for painful hot swollen skin, a rapidly spreading rash, fever with a rash, or shingles near the eye. Arrange a GP or pharmacy review for persistent, recurring or unexplained rashes. Seek earlier advice for babies, pregnancy, immune suppression or a new medicine related rash. This lesson explains how clinicians describe and assess rashes. It cannot diagnose an individual skin condition or replace urgent medical care.

A rash is described through morphology, distribution, evolution, symptoms and blanching. These findings guide probability and urgency, but no single feature identifies the diagnosis.