Innovative Methodology A new method for determining plasma water content: application in pseudohyponatremia

Nguyen MK, Ornekian V, Butch AW, Kurtz I. A new method for determining plasma water content: application in pseudohyponatremia. Am J Physiol Renal Physiol 292: F1652–F1656, 2007. First published February 13, 2007; doi:10.1152/ajprenal.00493.2006.—Pseudohyponatremia is a clinical condition characterized by an increased fraction of protein or lipid in plasma, thereby resulting in an artificially low plasma sodium concentration ([Na ]p). Since the automated method of measuring [Na ]p in most laboratories involves the use of an indirect ion-selective electrode (I-ISE), this method does not correct for elevated protein or lipid concentrations. In I-ISE, the plasma sample is diluted before the actual measurement is obtained, and the [Na ]p is determined based on the assumption that plasma is normally composed of 93% plasma water. Therefore, the [Na ]p as determined by I-ISE will be artificially low in clinical conditions when the plasma water content (PWC) is 93%. In contrast, the plasma is not diluted when the [Na ]p is measured using direct ISE (D-ISE). This method directly measures Na activity in plasma water and is therefore unaffected by the proportion of plasma occupied by water. In this study, we report a novel quantitative method for determining the PWC utilizing I-ISE and D-ISE. To validate this new method experimentally, we altered the PWC in vitro by dissolving varying amount of salt-free albumin in human plasma. We then measured PWC gravimetrically in each sample and compared the gravimetrically determined PWC with the ISE-determined PWC. Our findings indicate that the PWC can be accurately determined based on differences in the [Na ]p as measured by I-ISE and D-ISE and that this new quantitative method can be a useful adjunct in the analysis of the dysnatremias.