RCGP SCA Algorithm — UK Primary Care
Look at the whole skin, the mouth, eyes and genitals, and take observations before naming the rash. Painful skin, mucosal erosions, a recent new drug or a systemically unwell patient make it an emergency. Non-blistering maculopapular rashes: see the maculopapular rash algorithm; itch without rash: widespread itch.
SJS/TEN carries a substantial mortality that rises with the area of skin detachment and with delay in stopping the culprit drug; early withdrawal of the drug and transfer to a unit used to managing skin failure are the two interventions that matter (BAD 2016). Skin pain out of proportion to the appearance, mucosal involvement at two or more sites and a positive Nikolsky sign are the clues.
Erythroderma, from any cause, is "acute skin failure" with risks of hypothermia, fluid and protein loss, high-output cardiac failure and sepsis. Eczema herpeticum can be fatal and causes herpetic keratitis; antiviral treatment should not wait for swab results.
The drug history decides more emergencies here than any test. Record every drug started in the previous 8 weeks with dates, because the latency between starting a drug and the reaction points to the culprit and the type of reaction.
The level of the split in the skin explains the clinical appearance: subepidermal blisters (pemphigoid, dermatitis herpetiformis) are tense and stay intact; intraepidermal blisters (pemphigus, impetigo) are fragile and quickly become erosions. Mucosal involvement and a positive Nikolsky sign push the differential towards the dangerous conditions.
Most blistering diagnoses are confirmed by a dermatology biopsy, so the GP's tests are aimed at safety (organ involvement in DRESS, eosinophilia, renal function), at treatable infections (HSV and VZV PCR) and at the associations that change management (coeliac disease, syphilis, HIV).
In older patients the commonest blistering disease is bullous pemphigoid, which often starts with weeks of itch and urticated plaques before blisters appear and is often labelled eczema or scabies. Pemphigus is rarer but more dangerous and usually involves the mouth. Secondary syphilis is "the great imitator" and should be considered in any unexplained widespread rash, particularly with palm and sole involvement.
Severe cutaneous adverse reactions should also be reported to the MHRA through the Yellow Card scheme, and the culprit drug recorded prominently as an allergy in the record. Dermatology departments generally offer same-day telephone or photo advice for suspected immunobullous disease, which is often the quickest route.
Next steps: see Bullous pemphigoid (and pemphigus) · Urticaria · Atopic eczema · Psoriasis · Pityriasis rosea · Shingles · Impetigo · Coeliac disease · Anaphylaxis
Stopping the culprit drug early is the single intervention that most improves outcome in severe drug reactions. Topical clobetasol is effective and safer than oral prednisolone for localised bullous pemphigoid in frail older people (BAD 2012), but the diagnosis should be confirmed by dermatology. Oral antivirals for chickenpox in adults are most useful within 24 hours of the rash; eczema herpeticum is treated on clinical suspicion.
UKHSA advises exclusion from school or work for chickenpox until all vesicles have crusted, and for impetigo until lesions are crusted or 48 hours after starting antibiotics. People recovering from SJS/TEN or DRESS must never be re-exposed to the culprit or closely related drugs.
SJS/TEN and DRESS can evolve over days, so a patient who seems mildly unwell with an early drug rash needs a short review interval and explicit warning signs. Photographs alone miss mucosal involvement and the Nikolsky sign.