Reasoning GP · Algorithm · 1-page summary
Widespread or blistering rash in adults
For: adults with a new widespread or blistering rash in primary care. Not for: children (children's pathways); localised single lesions; non-blistering maculopapular rashes (maculopapular rash algorithm).
Core rule: look at the mouth, eyes and genitals, take observations and list every drug started in the last 8 weeks before naming the rash. Painful skin with mucosal erosions is SJS/TEN until proven otherwise.
0Stop — call 999 now
- Mucosal erosions + painful dusky or peeling skin after a new drug (SJS/TEN) → stop drug, 999 or same-day emergency admission
- Non-blanching rash, unwell → 999; benzylpenicillin or ceftriaxone if no delay to transfer
- Pain out of proportion, necrotic skin (necrotising infection) → 999
- Airway, breathing or circulation compromise with wheals → IM adrenaline 500 micrograms, 999
1Triage — the blister and the patient
| Finding | Think |
|---|---|
| Flaccid + mucosa | SJS/TEN, pemphigus Same-day |
| Tense, itchy, older | Bullous pemphigoid → dermatology |
| Grouped vesicles | HSV, zoster, dermatitis herpetiformis |
| Crops, fever | Chickenpox; pregnant/immunosuppressed Same-day |
| Wheals | Under 24 h urticaria; over 24 h + bruising: vasculitis |
2Screen
- Erythroderma (90% or more red skin) → Same-day
- Eczema herpeticum → aciclovir now + Same-day dermatology
- DRESS: fever, facial swelling, nodes 2–8 weeks after a drug → Same-day
- Disseminated zoster → Same-day
- Palms and soles, sexual risk → syphilis/HIV tests; sexual health within a week
3Assess
☐ Obs, NEWS2 ☐ Mouth, eyes, genitals ☐ Nikolsky sign ☐ Blister type, distribution, % body surface ☐ Drug list with start dates ☐ Bloods if unwell (FBC + eosinophils, LFT, U&E) ☐ HSV/VZV PCR swab ☐ tTG IgA if dermatitis herpetiformis
Do not start oral steroids for an undiagnosed blistering rash without specialist advice.
4Classify
- Severe drug reaction — new drug, fever, painful skin → admit.
- Immunobullous — pemphigoid (tense, older), pemphigus (erosions, mouth), dermatitis herpetiformis.
- Viral — chickenpox, zoster, HSV, mpox.
- Urticaria / urticarial vasculitis.
- Papulosquamous — guttate psoriasis, pityriasis rosea, secondary syphilis.
- Uncertain — photo to dermatology; review in 48–72 hours.
5Treat today — doses: NICE/SPC, BNF, local formulary
- Stop the culprit drug; record allergy; Yellow Card.
- Eczema herpeticum: aciclovir 400 mg five times a day. Adult chickenpox within 24 h: 800 mg five times a day for 7 days.
- Pemphigoid (on advice): clobetasol 0.05% + emollient. Urticaria: cetirizine 10 mg daily.
- Bullous impetigo: flucloxacillin 500 mg four times a day for 5 days.
6Refer routinely
- Localised pemphigoid, dermatitis herpetiformis, urticarial vasculitis → Urgent dermatology.
- Extensive guttate psoriasis, atypical pityriasis rosea → dermatology.
7Never reassure by
- A normal temperature in early SJS/TEN or DRESS.
- A photo without looking in the mouth and eyes.
- "Eczema" or "scabies" in an itchy older patient with urticated plaques.
- "Infected eczema" when erosions are punched-out and painful.
- "Viral rash" on the palms and soles without syphilis serology.
- The drug was started "weeks ago" — DRESS takes 2–8 weeks.
8Safety-net — say it, then document it
999 “Call 999 if your skin starts peeling or blistering quickly, you get sore eyes with mouth or genital sores, a rash that doesn't fade under a glass, or swelling of the lips or difficulty breathing.” Same-day “Contact us today for a new fever, painful or spreading blisters, eye pain, facial swelling, or if you can't eat or drink.” Review in 48–72 hours.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies
Mucosa, eyes and Nikolsky checked
Drug list reviewed; culprit stopped
Dermatology/sexual health route agreed
Review in 48–72 h; safety-net given
Summary of the full Reasoning GP widespread or blistering rash algorithm, which holds the detail, examination, investigations and evidence. Sources: BAD guidelines (SJS/TEN, pemphigoid, pemphigus, urticaria); NICE NG240, NG253, NG153, NG20; UKHSA; BASHH; product SPCs. Clinical reference — follow local pathways; doses: NICE/SPC, BNF, local formulary.