Which equation is used to estimate extracellular fluid deficit in dehydration?

Which equation is used to estimate extracellular fluid deficit in dehydration?

• A. (0.5 × body weight in kg) × (serum sodium/140) − 1
• B. (1.0 × body weight in kg) × (serum sodium/140) − 2
• C. (0.25 × body weight in kg) × (serum sodium/140) − 0
• D. (0.5 × body weight in kg) × (serum sodium/140) − 0

Answer: (A)

Estimating extracellular fluid deficit uses a practical formula that ties together how big the patient is and how salty their blood is, because both factors determine how much water must move into the extracellular space to restore balance. The commonly used bedside estimate is: ECF deficit in liters ≈ 0.5 × body weight (kg) × (serum sodium / 140) − 1. The 0.5 × weight part reflects the approximate extracellular fluid space relative to body size, while the ratio of serum sodium to 140 adjusts for the osmolar load: higher sodium means more water is needed to dilute the extracellular compartment toward normal. Subtracting 1 liter serves to calibrate the estimate by accounting for a baseline amount of fluid that remains in circulation and isn’t counted among the deficit to be corrected in the initial calculation. For example, with a 10 kg child and a serum sodium of 150 mEq/L, the estimate is 0.5 × 10 × (150/140) − 1 ≈ 4.4 liters, guiding the fluid replacement plan. The other options use different multipliers or omit the subtraction, which would lead to implausible or inconsistent deficits and are not used in this approach.

Estimating extracellular fluid deficit uses a practical formula that ties together how big the patient is and how salty their blood is, because both factors determine how much water must move into the extracellular space to restore balance. The commonly used bedside estimate is: ECF deficit in liters ≈ 0.5 × body weight (kg) × (serum sodium / 140) − 1. The 0.5 × weight part reflects the approximate extracellular fluid space relative to body size, while the ratio of serum sodium to 140 adjusts for the osmolar load: higher sodium means more water is needed to dilute the extracellular compartment toward normal. Subtracting 1 liter serves to calibrate the estimate by accounting for a baseline amount of fluid that remains in circulation and isn’t counted among the deficit to be corrected in the initial calculation. For example, with a 10 kg child and a serum sodium of 150 mEq/L, the estimate is 0.5 × 10 × (150/140) − 1 ≈ 4.4 liters, guiding the fluid replacement plan. The other options use different multipliers or omit the subtraction, which would lead to implausible or inconsistent deficits and are not used in this approach.