Friday, April 18, 2014

Pseudohyponatremia: Is this really low sodium?

Periodically I get a call from a clinician to say, "what's wrong with your sodiums?" In general, this call is activated by a sodium<125 mmol/l.="" my="" first="" response="" question="" is="" always:="" what="" are="" the="" child’s="" protein="" and/or="" lipid="">

At issue is the type of ion-selective electrode (ISE) used to measure the electrolytes. There are two basic types: lists, indirect and direct, and know which one your Chemistry analyzer used is important. Direct ISEs are exactly that. They measure the ion activity in the sample directly, in which fluid volume present in the sample is, and basically not be affected by other components in the sample. The activity is then converted to concentration and a result is produced. Indirect ISE do not a direct measurement. They the sample first Verdun and measure the concentration of electrolytes in the diluted sample. This usually works well, but it is problematic when the sample happens to have a high concentration of proteins or lipids. The reason is this: systems that use ISE indirect measurement assume that the sample is all the water. In reality it is about 93% water with normal plasma/serum 7% solids present (proteins and lipids). If the sample is analyzed less than 93% water has, for example, when protein or lipid makes more than 7% of the volume, the resulting measurement will be falsely low, as you can see from this table. A normal, 7% solids sample that a indirect measurement of ISE you would give a value of 135 mmol/L; If the solid is 20%, you get that monster a value of 143 mmol/L.

This is called pseudohyponatremia. The sodium is not really low; It's perfectly normal. The instrument gives you a falsely low value. The vast majority of wet chemistry analyzers measure electrolytes by indirect ISE. Only a few great chemistry analyzers measuring using direct ISEs electrolytes, and which usually have a correction factor so that the measured directly results more in line with the vast majority of indirect ISE measurements are.

What can you do about improperly low hyperproteinemia or sodiums caused by Hyperlipidemia? If it's related to lipids, you may clarify of the sample by centrifugation or chemicals and get a real result. Alternatively, blood gas analysers and some POC analyzers, such as the i-STAT, to measure electrolytes by direct ISE. If you or a whole blood sample can get, you can these analyzers to give you a real result. Else can you explain to a concerned doctor pseudohyponatremia.

-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the chemistry and metabolic disease laboratories in children's Medical Center in Dallas, TX and a Professor of Pathology at the University of Texas Southwestern Medical Center in Dallas.

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