Nguyen Minhtri K, Ornekian Vahram, Butch Anthony W, Kurtz Ira
Departments of Medicine, University of California, Los Angeles, California 90095, USA.
Am J Physiol Renal Physiol. 2007 May;292(5):F1652-6. doi: 10.1152/ajprenal.00493.2006. Epub 2007 Feb 13.
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(+)). Since the automated method of measuring Na(+) 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(+) is determined based on the assumption that plasma is normally composed of 93% plasma water. Therefore, the Na(+) 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(+) 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(+) 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.
假性低钠血症是一种临床病症，其特征为血浆中蛋白质或脂质比例增加，从而导致血浆钠浓度（Na⁺）人为降低。由于大多数实验室测量Na⁺的自动化方法涉及使用间接离子选择电极（I-ISE），该方法无法校正升高的蛋白质或脂质浓度。在I-ISE中，血浆样本在实际测量前被稀释，并且Na⁺是基于血浆通常由93%的血浆水组成这一假设来确定的。因此，当血浆含水量（PWC）<93%时，I-ISE测定的Na⁺在临床情况下会人为降低。相比之下，使用直接ISE（D-ISE）测量Na⁺时血浆不被稀释。这种方法直接测量血浆水中的Na⁺活性，因此不受水在血浆中所占比例的影响。在本研究中，我们报告了一种利用I-ISE和D-ISE测定PWC的新定量方法。为了通过实验验证这种新方法，我们通过在人血浆中溶解不同量的无盐白蛋白在体外改变PWC。然后我们用重量法测量每个样本中的PWC，并将重量法测定的PWC与ISE测定的PWC进行比较。我们的研究结果表明，基于I-ISE和D-ISE测量的Na⁺差异可以准确测定PWC，并且这种新的定量方法在分析钠代谢紊乱中可能是一种有用的辅助手段。
