Interpretation of U&Es


  • Sodium is commonly a marker of fluid balance (the major solute responsible for osmotic water movements)
    • Hypernatraemia
      • Usually secondary to dehydration.  Rarer causes include diabetes indipidus, salt poisoning
    • Hyponatraemia
      • Usually secondary to fluid overload (causing dilution of Na in the blood).  Often this can be a side effects of drugs, but other causes e.g. SIADH, may cause this


  • Potassium is crucial to the normal functioning of cells (particularly cells of the heart)
    • Hyperkalaemia is often secondary to drugs, haemolysis or renal impairment.
    • Hypokalaemia is often due to drugs, but can be secondary to potassium loss from the gut e.g. diarrhoea, or kidneys

Urea and Creatinine

  • Urea is a breakdown product of blood.  Creatinine is another breakdown product released by muscle.  They are both filtered by the kidneys.
  • A raised urea with a normal creatinine suggests a non-renal cause of hyperuraemia
    • Most commonly dehydration; more rarely, GI bleeds can cause a raised urea
  • A high urea with a raised creatinine suggests renal impairment (acute or chronic)

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