Erythema is a reddening of the skin due to persistent dilation of superficial blood vessels, and can be local or generalised. It is distinguished from flushing (see ‘Flushing’) by its permanence: Flushing is transient.

Published: 2nd August 2022 | Updated: 15th August 2022

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

  • Palmar Erythema (e.g. Pregnancy, Liver Disease, Thyrotoxicosis)
  • Phototoxic Reaction to Drugs (e.g. Phenothiazines, Tetracyclines, Diuretics)
  • ‘Deck-Chair Legs’ (Prolonged Immobility)
  • Erythema Multiforme (Various Causes)
  • Systemic Lupus Erythematosus (Erythematous, Photosensitive Butterfly Rash)
  • Erythema Ab Igne (Reticulate Pattern)

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)

Possible: FBC, ESR/CRP, LFT, TFT

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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Website disclaimer

Pulse Reference is based on the best-selling book Symptom Sorter.

The experts behind Pulse Reference are Dr Keith Hopcroft who is the co-author of Symptom Sorter, a GP in Essex and Pulse’s editorial advisor and Dr Poppy Freeman, a GP in Camden and also a clinical advisor to Pulse.

This website is for clinical guidance only and cannot give definitive diagnostic information. Practitioners should work within the limits of their individual professional practice, seek guidance when necessary and refer appropriately.