Water-deficit equation: Systematic analysis and improvement

Abstract

Background: The water-deficit equation {WD1 = 0.6 × B m × [1 - (140 ÷ Na+)]; Bm denotes body mass} is used in medicine and nutrition to estimate the volume (L) of water required to correct dehydration during the initial stages of fluid-replacement therapy. Several equation assumptions may limit its accuracy, but none have been systematically tested. Objectives: We quantified the potential error in WD 1 for the estimation of free water (FW) and total body water (TBW) losses and systematically evaluated its assumptions. Design: Thirty-six euhydrated volunteers were dehydrated (2.2-5.8% Bm) via thermoregulatory sweating. Assumptions within WD1 were tested by substituting measured euhydrated values for assumed or unknown values. These included the known (premorbid) Bm (WD2), a proposed correction for unknown Bm (WD3), the TBW estimated from body composition (WD4), the actual plasma sodium (WD5), the substitution of plasma osmolality (Posm) for sodium (WD6), and actual Posm (WD7). Results: Dehydration reduced TBW by 3.49 ± 0.91 L, 57% of which (2.02 ± 0.96 L) was FW loss, and increased plasma sodium from 139 (range: 135-143 mmol/L) to 143 (range: 141-148 mmol/L) mmol/L. Calculations for WD1 through WD7 all underestimated TBW loss by 1.5-2.5 L (P < 0.05). WD1 through WD5 underestimated FW by 0.5 L to 1.0 L (P < 0.05), but WD6 and WD7 estimated FW loss to within 0.06-0.16 L (P > 0.05). Conclusions: WD1 grossly underestimates TBW and FW losses. Corrections for unknowns and assumptions (WD2 through WD5) improved estimates little. The use of WD6 = 0.6 × B m × [1 - (290 ÷ Posm)] accurately estimates FW but still underestimates TBW losses by >40%. © 2013 American Society for Nutrition.