Water Deficit Calculator - Hypernatremia TBW Formula

A water deficit calculator turns body weight, the published body water fraction for the patient's age and sex, and a measured serum sodium from a basic metabolic panel into the free water deficit in liters, the standard screening aid for the workup of hypernatremia. The form returns the free water deficit alongside the safe correction time in hours.

• • Pre-rounds chart review: Enter the latest basic metabolic panel plus the patient weight before a shift so the conversation starts from a published free water deficit number.
• • Repeat scoring after a basic metabolic panel: Paste a fresh serum sodium plus the patient weight into the form after each set of labs and watch the deficit move as the measured sodium approaches the target.
• • Walk-through of the published worked example: Recheck the textbook example of a 70 kg adult man with a measured sodium of 147 mEq per liter and a target of 140 mEq per liter to see how the four inputs add up to about 2.1 L.

The water deficit calculator is most useful when the body weight and the serum sodium come from the same clinical picture, and the body water fraction and the sodium ratio are exposed as separate rows so the math can be sanity checked before any IV fluid goes in.

The total body water step is the same scaling factor used in the Body Water Calculator, which applies the more detailed Watson 1980 sex specific equations to the same body weight, age, and sex inputs for a paired reading.

The water deficit calculator first estimates total body water from the patient weight and the published body water fraction, then multiplies that total body water by the difference between the ratio of measured serum sodium to target serum sodium and 1.

• weight: Patient body weight in kilograms, converted from pounds at 0.45359237 kg per pound.
• ageSex: Age and sex category that picks the published body water fraction.
• measuredSodium: Measured serum sodium from a basic metabolic panel, in mEq per liter.
• desiredSodium: Target serum sodium in mEq per liter, with a default of 140 mEq per liter.

When the measured serum sodium is at or below the target, the calculator floors the free water deficit at zero and flags the result as no replacement needed, and hours to correct are computed at the published 0.5 and 1 mEq per liter per hour rates.

According to Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-9, the free water deficit is calculated as total body water times the difference between the ratio of measured serum sodium to desired serum sodium and 1, with a default desired sodium of 140 mEq per liter.

The same basic metabolic panel that drives this water deficit calculator also drives the Bicarbonate Deficit, and the two numbers are typically read together in any hypernatremia or metabolic acidosis workup.

Four concepts drive the result.

Chronic hypernatremia is generally defined as a serum sodium above 145 mEq per liter that has lasted more than 48 hours, and the brain adapts over that window so rapid correction can cause cerebral edema.

A sodium driven workup sits next to the basic metabolic panel in any electrolyte review, and the Anion Gap Calculator turns serum sodium, chloride, and bicarbonate into the gap that flags a masked acidosis.

The form works from a small set of basic metabolic panel values plus a single weight entry.

1. 1 Enter the patient weight and unit: Type the patient weight and pick kilograms or pounds.
2. 2 Pick the age and sex category: Choose the age and sex band that matches the patient. The body water fraction is 60 percent for adult males and children, 50 percent for adult females and elderly males, and 45 percent for elderly females.
3. 3 Enter the measured serum sodium: Type the measured serum sodium from the latest basic metabolic panel, in mEq per liter. A value above 145 mEq per liter is the typical trigger for a deficit calculation.
4. 4 Enter the target serum sodium: Type the target serum sodium in mEq per liter. The default of 140 mEq per liter sits inside the published reference range.
5. 5 Read the deficit and the safe correction time: The result panel shows the free water deficit in liters, total body water in liters, body water fraction, sodium ratio, hours to correct at 0.5 and 1 mEq per liter per hour, and a replacement needed flag.

A patient with a weight of 70 kg, an age and sex of adult male, a measured serum sodium of 147 mEq per liter, and a target of 140 mEq per liter enters those four numbers and gets a free water deficit of 2.1 L, a total body water of 42 L, a sodium ratio of 1.05, 14 hours to correct at 0.5 mEq per liter per hour, 7 hours to correct at 1 mEq per liter per hour, and a replacement needed flag.

The 60 percent child body water fraction sits in the same fluid balance family, and the Maintenance Fluids Children Calculator applies the Holliday Segar 4 2 1 rule to a child weight for a paired maintenance fluid order.

Using a water deficit calculator offers several practical advantages over mental math alone in any hypernatremia workup.

• • Standardized screening aid: Returns a single free water deficit in liters from a published body water fraction and the measured to target sodium ratio, so every member of the care team reads the same number.
• • Age and sex aware body water: Picks the published body water fraction for adult male, adult female, elderly male, elderly female, and child patients.
• • Pounds to kilograms conversion: Accepts the patient weight in kilograms or pounds, with pounds converted to kilograms at 0.45359237 kg per pound.
• • Safe correction time in hours: Pairs the free water deficit with the published chronic and acute correction rates of 0.5 and 1 mEq per liter per hour.
• • Replacement needed flag: Returns a yes or no replacement needed flag, so a measured sodium at or below the target reads as no replacement rather than a false positive deficit.

The output is a screening aid rather than a stand alone fluid order, and the calculator is most useful when the body weight and the serum sodium come from the same clinical picture.

A free water deficit is the reactive side of fluid management, and the Daily Water Intake Calculator is the daily preventive side, turning a body weight, activity level, and climate into a baseline hydration target.

The output depends on the basic metabolic panel values entered and on the patient.

• • The free water deficit is a screening aid, not a stand alone fluid order. The published chronic and acute correction rates assume a normal volume or low volume hypernatremia.
• • The body water fraction is a population average, so pregnant patients, patients with high body fat, and patients with severe muscle wasting can sit above or below the published fraction.

The body water fraction and the sodium ratio are exposed as separate rows in the result panel, so any unexpected move in the deficit can be traced to one of the two inputs.

According to Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-9, the free water deficit is the cornerstone of hypernatremia treatment and significant hypernatremia should be corrected slowly to prevent cerebral edema.

According to Medscape Hypernatremia reference, the normal serum sodium reference range is 135 to 145 mEq per liter and hypernatremia is generally defined as a serum sodium above 145 mEq per liter.

An arterial blood gas sits next to the basic metabolic panel in any hypernatremia workup, and the Acid Base Calculator reads pH, PCO2, and bicarbonate together for a complete acid base reading.