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  • Title: Whole-body electrolyte-free water clearance: derivation and clinical utility in analyzing the pathogenesis of the dysnatremias.
    Author: Nguyen MK, Kurtz I.
    Journal: Clin Exp Nephrol; 2006 Mar; 10(1):19-24. PubMed ID: 16544174.
    Abstract:
    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.
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