Erythema (for Flushing - see Flushing)

Differential Diagnosis

Common Diagnoses

  • Cellulitis
  • Gout
  • Burns: thermal, Chemical, Sunburn
  • Toxic Erythema: Drugs (e.g. Antibiotics, Nsaids), Bacteria (e.g. Scarlet Fever), Viruses (e.g. Measles, Slapped Cheek Syndrome)
  • Rosacea

Occasional Diagnoses

Rare Diagnoses

  • Fixed Drug Eruption
  • Livedo Reticularis: Connective Tissue Disease
  • Seroconversion Rash of HIV
  • Erythema Nodosum: Sarcoidosis, Streptococci, Tuberculosis, Drugs
  • Erythema Induratum (Bazin’s Disease: Tuberculosis)
  • Erythema Chronicum Migrans: Lyme Disease

Ready Reckoner

Key distinguishing features of the most common diagnoses

CellulitisGoutBurnsToxic ErythemaRosacea

Possible Investigations

Likely:Uric acid (if possible gout)


Small Print:Autoimmune studies, serology, CXR, ASO titre

  • FBC/ESR/CRP: WCC and ESR/CRP raised in significant infection; Hb may be reduced (normochromic normocytic) in connective tissue disorder
  • Autoimmune studies: If connective tissue disorder a possibility
  • Serology: May help if suspect infective cause for erythema multiforme; also useful in assessing immune status in a pregnant woman exposed to slapped cheek syndrome, and in diagnosis of HIV infection and Lyme disease (though current guidance states that a diagnosis of Lyme disease should be made clinically in patients with erythema migrans).
  • Uric acid: To confirm clinical suspicion of gout (when attack has subsided) especially if considering allopurinol.
  • LFT, TFT: If palmar erythema present in non-pregnant patient – to detect alcohol excess or hyperthyroidism.
  • Other investigations for erythema nodosum: If a non-drug cause is possible, investigations likely to include CXR (for TB, sarcoidosis) and ASO titre (for streptococcal infection).

Top Tips

  • Toxic erythema caused by drugs tends to be itchy; if due to infection, it does not irritate but is accompanied by fever.
  • Remember that there is often a delay before a drug causes toxic erythema – therefore, symptoms may only appear after a course of treatment (especially antibiotics) has been completed.
  • ‘Deck-chair legs’ is erythema of the lower legs, sometimes with oedema and blistering, in the immobile. It tends to be mistakenly diagnosed as persistent or recurrent cellulitis.
  • A violent local erythema, rapidly darkening and blistering and recurring at the same site, suggests a fixed drug eruption.
  • Remember to take a drug history, including over-the-counter medications. This may reveal the underlying cause in toxic erythema, erythema nodosum and multiforme, and phototoxicity.

Red Flags

  • Erythema nodosum and multiforme may be caused by significant disease, including, very occasionally, malignancy. If the patient is generally unwell or has other significant symptoms, investigate urgently or refer.
  • Take a travel history – lyme disease is endemic in forested areas. If not diagnosed and treated early, it can have significant complications.
  • Erythema multiforme with blistering and ulceration of the mucous membranes is Stevens– Johnson syndrome. Though rare, it is a very serious illness requiring urgent hospital treatment.
  • Enquire about joint symptoms – many causes of erythema (e.g. erythema multiforme, butterfly rash, livedo reticularis) can be linked to a connective tissue isorder
  • Remember that parvovirus can cause serious problems in pregnancy – check serology in women with suggestive symptoms, or exposure to a case.
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