Relationship of the absolute difference between direct and indirect ion selective electrode measurement of serum sodium and the total protein concentration

Aim: To derive the relationship for the difference between direct and indirect ion selective electrode measurements of serum sodium and the total protein concentration. Methods: Using modern analysers and independent specimens that covered the whole of the total protein range, linear relationships were derived for the difference between direct and indirect ion selective electrode measurements of serum sodium and the total protein concentration. Results: The regression data were as follows: absolute difference = 0.1196 TP - 5.9528, r(2) = 0.4555, p, 10248; relative difference = 0.0849 TP - 4.1199, r(2) = 0.4153, p < 10(-43). Conclusions: A linear regression equation for the relationship of the absolute difference between direct and indirect ion selective electrode measurements of serum sodium and the total protein concentration can be validly derived. However, due to the large spread of data around the regression line, such equations should not be employed to decide when to use direct electrodes instead of indirect electrodes in routine clinical laboratories.