Expansion
The volume of solute-free water excreted or retained per unit time
Expansion
Free water clearance = urine flow rate - osmolar clearance
Where osmolar clearance = (urine osmolality x urine flow) / plasma osmolality.
Interpretation:
- Positive: urine is hypotonic to plasma; the kidney is excreting free water. Seen in water loading and diabetes insipidus
- Zero: urine is isotonic
- Negative: urine is hypertonic; the kidney is conserving free water. Seen in dehydration and SIADH
The concept separates the excretion of solute from the excretion of water, which is the key to understanding sodium disorders.
Clinical corollary: a patient with SIADH has concentrated urine (negative free water clearance) despite hyponatraemia, which is inappropriate. A patient with appropriate physiology and hyponatraemia from water overload would have maximally dilute urine. Measuring urine osmolality in hyponatraemia therefore separates SIADH and other antidiuretic hormone-driven causes from primary polydipsia.