American Board of Surgery Qualifying Exam (ABS QE) Practice Test

How is free water deficit calculated?

1. (Serum Na - 140/140) x TBW

2. (140 - Serum Na) x TBW

3. (Serum Na + 140) x TWB

4. (140 - Serum Na) / TBW

The correct answer is: (Serum Na - 140/140) x TBW

The calculation of free water deficit is important in managing patients with hypernatremia or conditions associated with dehydration. The formula used indicates how the serum sodium concentration deviates from a normal baseline, which is typically considered to be 140 mEq/L. When calculating the free water deficit, you want to determine how much water (in liters) is needed to lower the serum sodium back to a normal level, starting from the existing serum sodium value. The correct approach to do this is by taking the difference between the observed serum sodium concentration and the normal value of 140. This difference is then divided by 140 to express the deviation as a fraction of the normal sodium level. Multiplying this fraction by the total body water (TBW) gives the total volume of free water needed to correct the hypernatremia. In essence, the formula effectively assesses the amount of excess sodium concentration present in the body compared to the standard, then calculates how much fluid is necessary to compensate for this excess sodium. This ensures the serum sodium level can return to a safe range without causing rapid shifts in serum osmolality, which could lead to complications like cerebral edema. Other options do not correctly reflect this process. For instance, they either represent the calculations in reversed