Pseudohyponatremia, as the name implies, is not what we consider true hyponatremia. In fact, it is a laboratory error (pseudo-greek derivation meaning false; feigned). When serum sodium is measured with the standard indirect ion-selective electrode (ISE) method it is measured in an aqueous solution, diluted, and there is always an assumption that plasma is precisely 93% water. When the samples are analyzed and something in the sample is “displacing” the plasma water, then the actual amount of sodium detected will be lower. What can displace the plasma water? Lipids, as in a very high triglyceride levels for starters. These elevated triglyceride levels, typically >1500 mg/dl, are often seen in conjunction with pancreatitis. Elevated proteins may also lead to Pseudohyponatremia; typically seen with diseases like Multiple Myeloma with total proteins in excess of 10 g/dL. Lastly, in severe biliary obstruction we may see very high levels of cholesterol and lipoproteins (lipoprotein x) usually with cholesterol levels over 1000 mg/dL.
This error can be avoided if a whole blood sample is drawn and analyzed with direct ion-selective electrode like the arterial blood gas method. Because this is not true hyponatremia, the treatment is directed at the underlying process and not on the low sodium.