Solute-Free versus Electrolyte-Free Water Clearance in the Analysis of Osmoregulation

Abstract

Background: Although attention has recently focused on electrolyte-free water clearance (E-C_H2O) as a replacement for solute-free water clearance (C_H2O), especially from the viewpoint of plasma sodium regulation, a thorough comparison of the two has yet to be conducted. Methods: C_H2O and E-C_H2O were systematically compared in normal subjects in different diuretic stages, including furosemide-induced solute diuresis, and in patients with renal disease. Results: The normal renal ability to conserve free water based on E-C_H2O was only 41% of that based on C_H2O. E-C_H2O remained positive until the urinary osmolality exceeded 500 mosm/kg H₂O, markedly different from the 300 mosm/kg H₂O for C_H2O. The difference between E-C_H2O and C_H2O could ultimately be attributed to urea osmolar clearance, i.e., urea excretion rate/plasma osmolality, which accounted for about 40% of the osmolar clearance. C_H2O underestimated the free water clearance by about 1 ml/min on average at all diuretic stages. Conclusions: E-C_H2O is a more correct parameter than C_H2O with regard to the regulation of both plasma sodium and plasma osmolality. However, there is the opinion that the concept of E-C_H2O is difficult to understand and that E-C_H2O is still not a generally accepted parameter. It is expected that the results of the present study will lead to more general acceptance.