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Free Water Excess Calculator

Formula

The formula to calculate the free water excess (FWE) is:

$FWE = \left( \frac{Na}{Na_{desired}} - 1 \right) \times W$

Where:

$FWE$ is the free water excess (liters).
$Na$ is the current sodium level (mmol/L).
$Na_{desired}$ is the desired sodium level (mmol/L).
$W$ is the weight of the patient (kg).

Example

Let's say the current sodium level is 140 mmol/L, the desired sodium level is 135 mmol/L, and the weight of the patient is 70 kg. Using the formula:

$FWE = \left( \frac{140}{135} - 1 \right) \times 70$

We get:

$FWE = \left( 1.037 - 1 \right) \times 70 = 0.037 \times 70 = 2.59$

So, the free water excess ( $FWE$ ) is 2.59 liters.

What is Free Water Excess?

Free water excess is a measure used in the medical field to determine the amount of water without solutes that needs to be removed or added to bring a patient’s sodium level to a desired target. It is particularly useful in the management of electrolyte imbalances such as hyponatremia, where the sodium concentration in the blood is abnormally low.