This calculator is intended to help you understand what is going on when you treat a patient with an abnormal serum sodium concentration. It can be used as an adjunct to your therapy but it is NOT a substitute for frequent measurements of the serum sodium concentration and adjustments in fluid management based on the results of those measurements.
The calculator makes use of observations made in the 1950’s that the serum sodium concentration (SNa) is a function of the body’s content of sodium and potassium divided by total body water (Edelman, JCI 37:1236-56, 1958).
SNa = Total Body (Na + K)/Total Body Water
After treatment the new SNa depends on Net Na + K Balance (input minus output) and Net Fluid Balance:
Net (Na+K) Balance = (Na+K) Intake - (Urine (Na+K) Concentration x Urine Volume)
Net Fluid Balance = Fluid intake - Urine Volume
Total body water is calculated using Watson’s formula which requires the following data:
• Patient’s sex
• Age in years
• Weight in lbs or kg
• Height in feet and inches or cm.
If exact values for any of these are unknown, it is better to enter an estimate than to leave anything blank.
Calculation of New SNa
To determine the new SNa, the calculator will sequentially estimate total body water, net Na + K balance and net fluid balance based on data that you enter. Urine output and urine electrolytes are key variables that are needed to determine net Na + K balance and net fluid balance. If urine output is unknown, it can be estimated from urine creatinine concentration and expected urine creatinine excretion based on the Cockroft-Gault formula (Dong Y, Silver SM, Sterns RH. Estimating urine volume from the urine creatinine concentration. Nephrology Dialysis Transplantation. 2021 Nov 25). If there are no data for urine parameters, urine volume will be assumed to be zero, and the predicted new SNa will be much less accurate.
Estimated Dose of 3% NaCl or D5W
To estimate how much 3% saline should be prescribed to increase the serum sodium concentration at the desired rate, vary the input of 3% saline until you see the serum sodium value that you want to achieve
To estimate how much D5W should be prescribed to relower the serum sodium concentration at the desired rate (in case of inadvertent overcorrection of hyponatremia) or to treat hypernatremia, vary the input of D5W until you see the serum sodium value that you want to achieve.