Whole-body electrolyte-free water clearance: derivation and clinical utility in analyzing the pathogenesis of the dysnatremias.

The total exchangeable sodium (Na(e)), total exchangeable potassium (K(e)), and total body water (TBW) are the major determinants of the plasma water sodium concentration ([Na(+)](pw)). The relationship between [Na(+)](pw) and Na(e), K(e), and TBW was empirically determined by Edelman et al., where: [Na(+)](pw) = 1.11(Na(e) + K(e))/TBW - 25.6 (Eq. 1). According to Eq. 1, changes in the mass balance of Na(+), K(+), and H(2)O will therefore result in changes in the [Na(+)](pw). Historically, in evaluating the pathogenesis of the dysnatremias, free water clearance (FWC) and electrolyte-free water clearance (EFWC) have been used to evaluate the pathophysiology of the dysnatremias. However, such analyses are only valid when there is no concomitant input and non-renal output of Na(+), K(+), and H(2)O. Since the classic FWC and EFWC formulas fail to account for the input and non-renal output of Na(+), K(+), and H(2)O, these formulas cannot be used to evaluate the pathogenesis of the dysnatremias or to predict the directional change in the [Na(+)](pw). In this article, we have addressed this limitation by deriving a new formula, termed whole-body electrolyte-free water clearance (WB-EFWC), which calculates whole-body electrolyte-free water clearance for a given mass balance of Na(+), K(+), and H(2)O, rather than simply the urinary component (FWC, EFWC formulas). Unlike previous formulas, which consider only the renal component of electrolyte-free water clearance, WB-EFWC accounts for all sources of input and output of Na(+), K(+), and H(2)O, and will therefore be helpful in conceptually understanding the basis for changes in the [Na(+)](pw) in patients with the dysnatremias.