The utility and accuracy of four equations in predicting sodium levels in dysnatremic patients

Background: Improper correction of hyponatremia can cause severe complications, including osmotic demyelination syndrome (ODS). The Adrogue-Madias equation (AM), the Barsoum-Levine (BL) equation, the Electrolyte Free Water Clearance (EFWC) equation and the Nguyen-Kurtz (NK) equation are four derived equations based on the empirically derived Edelman equation for predicting sodium at a later time (Na-2) froma known starting sodium (Na-1), fluid/electrolyte composition and input and output volumes. Methods: Our retrospective study included 43 data points from 31 mostly hyponatremic patients. We calculated Na-2 based on five sets of rules that were progressively more precisely calculated. Sets A-D included all 31 patients and 43 data points and set E was based on 15 patients and 27 data points. Results: The root mean square error was calculated and found to be between 4.79 and 6.37 mmol/L (mEq/L) for all sets. Bland-Altman analysis showed high variability and discrepancies between the predicted and actual Na-2. Conclusions: Like similar studies in hypernatremic patients, the data suggest that hyponatremic modeling equations are not reliably accurate in predicting Na-2 from Na-1 and available clinical data regarding sodium, potassium and fluid balance over longer time frames (12-30 h). Our study was retrospective and was done in an inpatient setting and thus was subject to limitations and laboratory measurement variability, but showed that all four equations are not able to reliably predict Na-2 from Na1 and inputs across a 12-30 h period.