Differences between CSF and plasma Na+ and K+ activities and concentrations.

[Na+] and [K+] determined by ion-selective electrodes (ISE) by direct potentiometric determination were compared with those obtained by flame photometry in plasma and in cisternal cerebrospinal fluid (CSF) obtained from pentobarbital-anesthetized dogs. Comparisons of pre- and postanesthesia values showed no change in plasma [Na+] but a 5% fall in plasma [K+]. CSF and plasma Na+ activities were identical, indicating that Na+ does not contribute to the potential difference between them. CSF K+ activity was 74% of that in plasma. The CSF-to-plasma molar and molal ratios differed from the activity (ISE) ratios. Concentrations of Na+ and K+ were also determined in an electrolyte solution and in the same solution with albumin added. Flame values differed from ISE values in plasma, CSF, and solutions. Albumin lowered flame values to a greater extent than ISE values. Bicarbonate lowered ISE values. ISE ratios, which equal activity ratios, rather than molar or molal ratios should be used in thermodynamic equilibrium equations, whereas molar concentrations should be used in electroneutrality equations or in determination of the strong ion difference.