II. Indications

  1. Quantitate renal concentrating function

III. Physiology

  1. Calculates the amount of free water to add or subtract from urine to make it isoosmotic with plasma

IV. Calculation

  1. cH2O = uV - (uOsm x uV)/pOsm
    1. where cH2O = Free Water Clearance
    2. where uV = Urine Volume per unit time
    3. where uOsm = Urine Osmolality
    4. where pOsm = plasma osmolality

V. Interpretation

  1. Free Water Clearance positive
    1. Urine less concentrated than plasma
    2. Decreased Antidiuretic Hormone (ADH)
  2. Free Water Clearance negative
    1. Urine is more concentrated than plasma
    2. Increased Antidiuretic Hormone (ADH)

VI. Resources

  1. Free Water Clearance (Wikipedia)
    1. https://en.wikipedia.org/wiki/Free_water_clearance

VII. References

  1. Goldberg (2014) Clinical Physiology, Medmaster, p. 34

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