Free Water Deficit in Hypernatremia

Sex

Age

years

Weight

Current Sodium

mEq/L

Calculates free water deficit by estimated total body water in a patient with hypernatremia or dehydration.

Why Use

This formula should be used to estimate the amount of free water that needs to be provided (in addition to ongoing free water needs) to correct an elevated serum sodium.

When to Use

This tool should be used when a patient is hypernatremic but not volume depleted.

Formula

Free water deficit, L = (% total body water, fraction)*(Weight, kg)*([Current sodium/Ideal sodium] – 1) where % total body water (TBW) is: Adult male: 60% (i.e., use 0.6 in the equation) Adult female: 50% (0.5) Elderly male: 50% (0.5) Elderly female: 45% (0.45) Child: 60% (0.6) This formula calculates the total solute in the patient’s total body water and then determines how much water needs to be added to dilute it to the desired serum sodium. Sufficient free water should be provided either orally or intravenously (e.g., 5% dextrose) to correct the serum sodium by up to 10 mEq/L in the first 24 hours.

Pearls / Pitfalls

This tool provides an estimate of free water deficit based on a patient's body weight; however, it may be inaccurate in patients with significant weight gain or loss (especially from fluid sources).

Management

Sufficient free water should be provided either orally or intravenously (using a solution such as 5% dextrose) to correct the serum sodium by up to 10 mEq/L in the first 24 hours.

Advice

Hypernatremia is an elevation in serum sodium, indicating a free water deficit or dehydration. In addition to calculating the free water deficit, the cause should be determined (e.g., loss of free water in the urine or stool, or lack of access to sufficient free water). Patients may simultaneously be volume depleted and dehydrated. If this is suspected, correct the volume deficit with isotonic fluids first before correcting the water deficit calculated here.