Direct potentiometric analysis of sodium and potassium in human plasma: evidence for electrolyte interaction with a nonprotein, protein associated substance(s)

A new instrument (Orion Model SS/30) designed to measure sodium and potassium by potentiometry without sample dilution (direct potentiometry) was found to be linear, precise, and free from interference and to have carryover. Unlike flame photometry or potentiometry performed after sample dilution, direct potentiometry was unaffected by changes in plasma water caused by lipemia or changes in protein concentration. Studies with deionized bovine or human sera suggested that the ratio of flame photometry results to direct potentiometry results should be approximately equal to plasma water (~92%) However, studies in 261 patients indicated that the actual ratios were 98.8% ± 1.3 for sodium and 96.0% ± 2.0 for potassium. These results were confirmed by studies in patients with a broad range of protein levels. The results suggest an interaction of sodium with components in sera of ~7% and potassium of ~4%. Further evidence for this interaction was obtained by hydrogen ion-induced increases in free sodium and potassium in pooled sera below pH 5. The substance(s) responsible for the interaction with sodium and potassium were not present in plasma ultrafiltrates. It appears that sodium and potassium in human plasma are associated with substances associated with proteins but not to the proteins themselves. The substances responsible and their physiological significance are not yet known. Direct potentiometry will be a valuable investigative tool for the study of electrolyte physiology. Clinically, it should prove particularly useful in conditions associated with elevated protein or lipid values.