Kim Gheun-Ho
Department of Internal Medicine and Institute of Biomedical Sciences, Hanyang University College of Medicine, Seoul, Korea.
Electrolyte Blood Press. 2006 Nov;4(2):77-82. doi: 10.5049/EBP.2006.4.2.77.
Serum consists of water (93% of serum volume) and nonaqueous components, mainly lipids and proteins (7% of serum volume). Sodium is restricted to serum water. In states of hyperproteinemia or hyperlipidemia, there is an increased mass of the nonaqueous components of serum and a concomitant decrease in the proportion of serum composed of water. Thus, pseudohyponatremia results because the flame photometry method measures sodium concentration in whole plasma. A sodium-selective electrode gives the true, physiologically pertinent sodium concentration because it measures sodium activity in serum water. Whereas the serum sample is diluted in indirect potentiometry, the sample is not diluted in direct potentiometry. Because only direct reading gives an accurate concentration, we suspect that indirect potentiometry which many hospital laboratories are now using may mislead us to confusion in interpreting the serum sodium data. However, it seems that indirect potentiometry very rarely gives us discernibly low serum sodium levels in cases with hyperproteinemia and hyperlipidemia. As long as small margins of errors are kept in mind of clinicians when serum sodium is measured from the patients with hyperproteinemia or hyperlipidemia, the present methods for measuring sodium concentration in serum by indirect sodium-selective electrode potentiometry could be maintained in the clinical practice.
血清由水（占血清体积的93%）和非水成分组成，主要是脂质和蛋白质（占血清体积的7%）。钠局限于血清中的水。在高蛋白血症或高脂血症状态下，血清中非水成分的质量增加，而由水组成的血清比例随之降低。因此，由于火焰光度法测量的是全血浆中的钠浓度，会导致假性低钠血症。钠选择性电极能给出真实的、生理相关的钠浓度，因为它测量的是血清水中的钠活性。在间接电位法中血清样本会被稀释，而在直接电位法中样本不会被稀释。因为只有直接读数能给出准确的浓度，我们怀疑许多医院实验室目前使用的间接电位法可能会在解释血清钠数据时误导我们，导致混淆。然而，在高蛋白血症和高脂血症病例中，间接电位法似乎很少会给出明显偏低的血清钠水平。只要临床医生在测量高蛋白血症或高脂血症患者的血清钠时记住小误差范围，则目前通过间接钠选择性电极电位法测量血清钠浓度的方法在临床实践中仍可维持使用。
