The effect of albumin concentration on plasma sodium and chloride measurements in critically ill patients.

Background: We tested the hypothesis that the difference between indirect and direct sodium assays would be related to the plasma albumin concentration. Further, we proposed that differences between indirect and direct chloride assays might be explained by interference from other plasma constituents, particularly bicarbonate, and possibly albumin.
Methods: We studied 300 critically ill patients at the time of admission to the intensive care unit (ICU) and compared each patient's plasma sodium and chloride measurements from a central laboratory assay (indirect electrode) and an ICU blood gas machine assay (direct electrode).
Results: The central laboratory sodium measurement was, on average, 2.1 mmol/L more than the ICU assay, limits of agreement 1.8-2.4 mmol/L greater, P < 0.001. The central laboratory chloride measurement was, on average, 1 mmol/L less than the ICU assay (limits of agreement 1.3-0.7 mmol/L less, P < 0.001). All correlations between the assay differences and plasma constituents were weak except for a moderately strong correlation between differences in sodium measurements and albumin. The difference in plasma sodium concentration between the assays (central laboratory - ICU) increased as the plasma concentration albumin decreased (difference = 6.2-0.16 albumin (g/L); P < 0.001, r = -0.46, r(2) = 0.22).
Conclusions: The central laboratory and ICUs assays are analytically, statistically, and clinically different for both sodium and chloride. Unless taken into account, the differences could be large enough in hypoalbuminemic populations (such as critically ill patients) to affect clinical diagnosis and decision making.