Thread by @sargsyanz

In hypernatremia, it’s common to estimate the free water deficit. A shortcut in an average-sized person is (Serum Na - 140) / 3 = liters of deficit.

Na is 146? (146-140)/3 = 2L FWD.

BUT, consider...

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That when you are hypovolemic, your serum Na can either be high (if you don't drink to replace the H2O you lose) or low (if you replace H2O > Na).

And anywhere in between!

So if the balance falls >140, and your doc’s estimating a deficit, that’s only FREE water deficit.

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The patient may have an additional isotonic fluid deficit.

That person with Na of 146 may actually be 8 liters down.

6 liters of isotonic salt water

+ 2 liters of just water.

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