Only a few big chemistry analyzers measure electrolytes using direct ISEs, and those usually have a correction factor so that the directly measured results are more in line with the big majority of indirect ISE measurements. The vast majority of wet chemistry analyzers measure electrolytes by indirect ISE. The instrument is giving you a falsely low value. The sodium is not really low it’s perfectly normal. A normal, 7% solids sample that an indirect ISE measurement would give you a value of 135 mmol/L if the solids are 20%, that sample will give you a value of 116 mmol/L. If the sample being analyzed has less than 93% water, for example when either protein or lipid makes up more than 7% of the volume, the resulting measurement will be falsely low, as you can see from this table. In reality, normal plasma/serum is roughly 93% water with 7% solids present (proteins and lipids). The reason for that is this: systems using indirect ISE measurement assume that the sample is all water. This usually works well, but becomes problematic when the sample happens to have a high concentration of proteins or lipids. They dilute the sample first and measure the concentration of electrolytes in the diluted sample. Indirect ISE’s do not do a direct measurement. The activity is then converted to concentration and a result is produced. They measure the ion activity in the sample directly, in whatever fluid volume is present in the sample, and are basically not affected by other constituents in the sample. There are two basic types, indirect and direct, and knowing which one your chemistry analyzer uses is important. My first response question is always: What are the child’s protein and/or lipid levels?Īt issue here is the type of ion-selective electrode (ISE) used to measure the electrolytes. Periodically I get a call from a clinician saying, “What’s wrong with your sodiums?” In general, this call is triggered by a sodium <125 mmol/L.
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