In favor of point-of-care sodium measurement

When monitoring the plasma/serum sodium concentration of sick newborn babies in neonatal intensive care units, it may be preferable to use direct ion-specific electrode (ISE) methodology incorporated in point-of-care analyzers (including blood gas analyzers), rather than the indirect ISE methodology commonly employed in central laboratory analyzers. 

That is the headline conclusion of a study that was prompted by the finding of marked discrepancy between sodium results for a sick neonate in the study authors’ care; plasma sodium generated at the point of care was 123 mmol/L, whereas a concurrent sample sent to the laboratory returned a value of 134 mmol/L.

Further testing revealed a marked reduction in the baby’s plasma albumin concentration (26 g/L), and the discrepancy in sodium values was attributed to a well-documented effect that serum protein concentration has on sodium concentration when measured using the indirect ISE methodology employed in clinical laboratories. 

This effect (known as the ”ion exclusion” effect) results in falsely low plasma sodium (pseudohyponatremia) if plasma protein (or lipid) concentration is markedly increased, and falsely increased plasma sodium concentration (pseudohypernatremia) if plasma protein concentration is markedly reduced. Methods of sodium measurement that employ direct ISE are unaffected by protein concentration.

Those caring for the baby decided to investigate further to establish the frequency of reduced albumin among sick neonates and confirm the putative correlation between albumin concentration and the magnitude of difference between the two sodium methodologies. 

From their laboratory database of neonatal sodium estimations over a 4-year period, they recovered 2420 matched pairs of sodium result, one obtained by direct ISE at the point of care, and the other obtained within 30 minutes by indirect ISE measurement at the central laboratory. For each of these pairs they recovered a concurrent plasma albumin value. 

Analysis of the data revealed that hypoalbuminemia (defined as albumin <30 g/L) is common in sick neonates; it was apparent in 1400 of the 2420 (58 %) paired results. The difference between direct and indirect ISE sodium results increased as albumin concentration decreased. 

For 31 % of pairs there was a clinically significant difference of more than 3 mmol/L. Hyponatremia (Na <135 mmol/L) was correctly diagnosed by direct ISE in 758 samples but in only 531 samples by indirect ISE. 

The authors of the study conclude that because of the frequency of hypoalbuminemia among sick neonates and the potential for clinically significant effect of hypoalbuminemia on sodium values when measured by indirect ISE, it would be preferable to use direct ISE when assessing sodium balance in the neonatal intensive care unit. This recommendation is in line with advice from the IFCC.