Nephrology
Free Water Deficit
Calculates the free water deficit in hypernatremia.
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Parameters
Free Water Deficit Result
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Formula
Water Deficit (L) = TBW × [(Measured Na / Target Na) - 1]
TBW Factors:
Male < 65 yrs: 0.6
Male ≥ 65 yrs: 0.5
Female < 65 yrs: 0.5
Female ≥ 65 yrs: 0.45Clinical Pearls
Provides the volume of free water (usually administered as D5W, enteral water, or 0.45% NaCl) needed to return serum sodium to normal.
Limitations
Assumes that sodium mass remains constant. Does not account for ongoing sensible and insensible fluid losses, which must be added to the calculated deficit during replacement.