Measurements of sodium and potassium in newborns and prematures by ion-selective electrodes and flame photometry: influence of lipid and protein content of blood.

The alternative use of flame photometry and ion selective electrodes for electrolyte determinations in neonatology led to the question how the two methods agree. 104 routine blood samples were taken as matched pairs for a comparison of sodium and potassium determinations. Pre-analytical error was avoided by the mode of blood collection. The influence of the protein and lipid content of blood on the differences between the methods was determined by calculating its water content and making a post-correction of the flame values. Thus the differences for sodium could be reduced, whereas the differences for potassium slightly increased. As long as the imprecision of sodium determination is a meaningful methodological problem, the influence of the protein and lipid content of neonatal blood is not the main factor for differences between ISE and flame photometry.