II. Indications
- Quantitate renal concentrating function
III. Physiology
- Calculates the amount of free water to add or subtract from urine to make it isoosmotic with plasma
IV. Calculation
- cH2O = uV - (uOsm x uV)/pOsm
- where cH2O = Free Water Clearance
- where uV = Urine Volume per unit time
- where uOsm = Urine Osmolality
- where pOsm = plasma osmolality
V. Interpretation
- Free Water Clearance positive
- Urine less concentrated than plasma
- Decreased Antidiuretic Hormone (ADH)
- Free Water Clearance negative
- Urine is more concentrated than plasma
- Increased Antidiuretic Hormone (ADH)
VI. Resources
- Free Water Clearance (Wikipedia)
VII. References
- Goldberg (2014) Clinical Physiology, Medmaster, p. 34