Sodium and risk assessment of osmotic demyelination syndrome: the method matters!

What is it about?

Sodium concentrations are determined by blood gas analyzers and chemistry analyzers via direct and indirect ion-selective electrodes. Sodium concentrations obtained via both methods are not always concordant. In critically ill patients, total protein concentrations may be abnormal. In patients in whom the plasma non-water fraction is increased, sodium may be underestimated when using indirect sodium electrodes. As the sodium reference range is narrow and the biological variation is low, effects of hypo-or hyperproteinemia on sodium results cannot be neglected. We read the article by Sicker et al, dealing with a clinical decision support system to identify patients with profound hyponatremia at risk of delayed control measurements or inadequate fast correction. In their study, it appears that the above mentioned caveat is overlooked, thereby assuming a mono-compartment distribution model. In more than one third of intensive care unit patients sodium results obtained by indirect potentiometry give falsely elevated sodium values. The difference between sodium direct and indirect results is explained by hypoproteinemia. One needs to rely rather on direct or lab correct sodium concentrations. Depending on the method, the difference between consecutive sodium concentrations may require an intervention based on the algorithm as presented by Sicker et al. Sodium results obtained from indirect and direct potentiometry are not interchangeable in case of abnormal protein concentration. As direct sodium measurements are not influenced by the total protein concentration, they should be preferentially used in patients with abnormal protein concentrations. Correcting sodium values for differences in co-volume is recommended in critical care patients. Clinicians need to be aware about the sodium method used, especially when consecutive sodium concentrations are determined.

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