title: A new method for determining plasma water content: application in pseudohyponatremia.
abstract: 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.
authors.affiliation: Departments of Medicine, University of California, Los Angeles, California 90095, USA. mtnguyen@mednet.ucla.edu
authors.forename: Minhtri K
authors.initials: MK
authors.lastname: Nguyen
authors.forename: Vahram
authors.initials: V
authors.lastname: Ornekian
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Butch
authors.forename: Ira
authors.initials: I
authors.lastname: Kurtz
mesh_terms: D001774:Blood Chemical Analysis / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D001798:Blood Proteins / Q000378:metabolism*; D001834:Body Water / Q000378:metabolism*; D006801:Humans; D007010:Hyponatremia / Q000097:blood*; D066298:In Vitro Techniques; D017736:Ion-Selective Electrodes; D008055:Lipids / Q000097:blood*; D009994:Osmolar Concentration; D012964:Sodium / Q000097:blood
chemical_list: D001798:Blood Proteins; D008055:Lipids; D012964:Sodium