Case of the Month #48 Nephrogenic DI

- The most commonly used formula of calculating serum osmolality is:  

2Na + Glucose + BUN (All in mmols) 

- To calculate the water deficit, we must calculate the current Total Body Water using the following equation: 

Current (TBW × Na) = Normal (TBW x Na) 

Considering that:  

• Normal Na = 140 

• TBW is usually 60% of lean Body Weight in Men and 50% in Women. 

The equation can be modified: 

Current TBW = Normal TBW × (140/Current Na) 

Once the current TBW is calculated, the free water deficit is calculated as:  

Free Water Deficit (L) = Normal TBW – Current TBW 

The estimated free water deficit should be corrected over 48 to 72 hours with a decrease in serum sodium not exceeding 0.5 mmol /l/hour. Patients should be carefully monitored for the rate of correction, urine output, and ongoing losses.