Urine volume

Urine volume is a poor guide to renal function as it is physiologically variable and 24-hour collections are often erroneous. However severe oliguria or polyuria imply pathology and require further investigation.

Too much

Difficult to set an upper limit of normal, but 4 litres is a lot.  Causes:

  • Osmotic diuresis – diabetes mellitus
  • Loss of antidiuretic hormone (ADH) – cranial diabetes insipidus (head injury, etc)
  • Failure to respond to ADH – nephrogenic diabetes insipidus
  • Excessive intake – psychogenic polydipsia

Too little

300-500 mls of urine is needed to excrete adequate amounts of solute each day.  Less than this is therefore oliguria.

  • Physiological response to severe dehydration
  • Hypoperfusion of the kidneys – hypotension or damage/occlusion to renal artery/aorta
  • Acute Renal Failure (see Approach to the patient with oliguria)