An Approach to the Unknown Rash

Author: Michelle Lin, MD
Updated: 8/26/2011

Rashes: An Approach to the Unknown Rash

How to diagnosis a rash

  1. Accurately describe the rash (eg. macule or nodule, vesicle, or bulla)
  2. Quickly "profile" the rash to see if it fits any classic pattern by patient age, rash distribution, or presence of hypotension
  3. Apply an algorithm based on the rash type

Rash Descriptors

Descriptor Definition
Lesion Single small diseased area
Rash Eruption of skin; more than single lesion
Macule Circumscribed area of change without elevation
Papule Solid raised lesion <1 cm
Nodule Solid raised ≥1 cm
Plaque Circumscribed elevated confluence of papules ≥1 cm
Pustule Circumscribed area containing pus
Vesicle Circumscribed fluid-filled area <1 cm
Bulla Circumscribed fluid-filled area ≥1 cm
Petechia Small red/brown macule ≤1 cm that does not blanch

Quick Profiling of Rash

Patient Age

0-5 years

  • Meningococcemia
  • Kawasaki disease
  • Viral exanthema

> 65 years

  • Pemphigus vulgaris
  • Sepsis
  • Menigococcemia
  • Toxic epidermal necrolysis
  • Stevens Johnson syndrome
  • Toxic shock syndrome


Diffuse Erythema

  • Staph scalded skin syndrome
  • Staph/strep toxic shock syndrome
  • Necrotizing fasciitis

Mucosal Lesions

  • Erythema ultiforme major
  • Toxic epidermal necrolysis/Stevens Johnson syndrome
  • Pemphigus vulgaris


  • Menigococcemia
  • Necrotizing fasciitis
  • Vasculitis
  • Disseminated intravascular coagulopathy
  • Rocky Mountain Spotted Fever



  • Meningococcemia
  • Toxic shock syndrome
  • Rocky Mountain Spotted Fever
  • Toxic epidermal necrolysis
  • Stevens Johnson syndrome

Erythematous Rash

Erythematous rash workup pathway

Maculopapular Rash

Maculopapular rash workup pathway

Petechial/ Purpuric Rash

Petechial or purpuric rash workup pathway

Vesiculobullous Rash

Vesiculobullous rash workup pathway


  • Murphy-Lavoie H, Le Gros TL. Emergent Diagnosis of the Unknown Rash. Emergency Medicine. 2010 March. PDF: [Source]