Differential Diagnosis

Occasional Diagnoses

Rare Diagnoses

  • Herpes Gestationis
  • Dermatitis Herpetiformis
  • Psychogenic (Includes Dermatitis Artefacta)
  • Leukaemia and Myeloproliferative Disorders

Ready Reckoner

Key distinguishing features of the most common diagnoses

Contact AllergyScabiesAtopic EczemaPityriasis RoseaPsoriasis
History of Chemical ExposureYesNoPossibleNoNo
Widespread ItchPossibleYesPossibleYesNo
Worse at NightNoYesPossibleNoNo
Mainly on FlexuresNoNoYesNoNo
Mainly on Extensor SurfacesPossibleNoNoNoYes

Possible Investigations


Possible:Urinalysis, blood glucose or HbA1c, FBC, ferritin, ESR, U&E, LFT, TFT.

Small Print:None.

  • Urine: Dipstick for glycosuria (blood glucose or HbA1c if positive).
  • FBC, ferritin: Will reveal iron-deficiency anaemia and polycythaemia; eosinophil count may be raised in allergic conditions; WCC may be very high in leukaemia; ESR may be elevated in lymphoma; low ferritin will confirm iron deficiency.
  • U&E: Will reveal uraemia.
  • LFT: Deranged liver enzymes and raised bilirubin in liver disease. Abnormalities here will require further investigations or referral.
  • TFT: Both hypo- and hyperthyroidism can lead to skin changes which cause itching.

Top Tips

  • The key to sorting out this symptom is to identify whether or not there is an underlying rash. Itching with a rash depends on identifying the rash; itching without a rash (other than that caused by scratching) depends on identifying any systemic cause.
  • An itchy, unidentifiable rash which is worse at night is likely to be scabies, particularly if any contacts are affected.
  • Warn the patient that scabies treatment may take a week or two fully to relieve symptoms – otherwise, the patient may apply the treatment repeatedly, causing a chemical irritation and diagnostic confusion.
  • The books usually state that psoriasis doesn’t itch – but it certainly can, so don’t let this symptom put you off the diagnosis.
  • It is usually very difficult to identify the allergen in a single episode of urticaria (and, indeed, the cause may not be allergic at all). Tell the patient to keep a note of foods or medicines just ingested so that, in the event of recurrence, any culprit can be identified.

Red Flags

  • If no obvious cause, always examine the abdomen and lymph nodes – do not miss lymphadenopathy, or enlarged liver, spleen or kidneys.
  • Don’t be tempted not to examine the itchy, malodorous self-neglected patient – poor personal hygiene may deceptively mask some other identifiable underlying cause.
  • Beware of apparently florid eczema appearing for the first time in an elderly patient – this may be a manifestation of serious underlying pathology.
  • Don’t forget iatrogenic causes – enquire about any drugs recently prescribed.
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