Londoño et al (Gut 2007;56:1283–90) report data from a retrospective cohort study and conclude that serum sodium is an independent predictor of survival for patients awaiting liver transplantation. We wish to highlight two issues that clinicians should be aware of before using serum sodium in this way.

First, two method types are commonly employed in modern hospital laboratories to measure serum sodium: indirect and direct. Unfortunately, the method used by the study authors is not described. Measuring sodium indirectly, most commonly using an indirect ion-selective electrode …