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)
| Urine testing << | >> Vascular access |
