How to solve the underestimated problem of overestimated sodium results in the hypoproteinemic patient

Objectives: The availability of a fast and reliable sodium result is a prerequisite for the appropriate correction of a patient's fluid balance. Blood gas analyzers and core laboratory chemistry analyzers measure electrolytes via different ion-selective electrode methodology, that is, direct and indirect ion-selective electrodes, respectively. Sodium concentrations obtained via both methods are not always concordant. A comparison of results between both methods was performed, and the impact of the total protein concentration on the sodium concentration was investigated. Furthermore, we sought to develop an adjustment equation to correct between both ion-selective electrode methods. Design: A model was developed using a pilot study cohort (n = 290) and a retrospective patient cohort (n = 690), which was validated using a prospective patient cohort (4,006 samples). Setting: ICU and emergency department at Ghent University Hospital. Patients: Patient selection was based on the concurrent availability of routine blood gas Na-direct(+) as well as core laboratory Na-indirect(+) results. Interventions: In the pilot study, left-over blood gas syringes were collected for further laboratory analysis. Measurement and Main Results: There was a significant negative linear correlation between Na-indirect(+) and Na-direct(+) relative to changes in total protein concentration (Pearson r = -0.69; p < 0.0001). In our setting, for each change of 10 g/L in total protein concentration, a deviation of similar to 1.3 mmol/L is observed with the Na-indirect(+) result. Validity of our adjustment equation protein-corrected Na-indirect(+) = Na-indirect(+) - 10.53 + (0.1316 x total protein) was demonstrated on a prospective patient cohort. Conclusions: As Na-direct(+) measurements on a blood gas analyzer are not influenced by the total protein concentration in the sample, they should be preferentially used in patients with abnormal protein concentrations. However, as blood gas analyzers are not available at all clinical wards, the implementation of a protein-corrected sodium result might provide an acceptable alternative