Effects of ionic strength on ionized calcium measured using reference electrode liquid junctions of varying design...

Five commercial analysers were used to measure ionized calcium in aqueous and protein solutions which contained the same amount of calcium but had sodium chloride concentrations ranging from 100 to 150 mmol/L. In aqueous solutions measured activity fell significantly with increasing ionic strength while in protein solutions it increased. When isotonic sodium chloride was substituted for the hypertonic potassium chloride reference electrode liquid junction of an analyser with an open junction, there was a marked positive change in the effect of ionic strength in both aqueous and protein solutions. In contrast, when either isotonic sodium chloride or potassium chloride was substituted for the hypertonic potassium chloride of an instrument with a membrane-restricted junction, there was no effect on the change of measured values with ionic strength. Increasing the protein concentration by ultrafiltration did not change ionized calcium values when isotonic reference solutions were used with either open or membrane-restricted junctions. Because membrane-restricted isotonic junctions respond like hypertonic junctions to changes in sample ionic strength but do not exhibit protein interference, they may prove to have advantages over both open isotonic junctions and all configurations of hypertonic junction for measurements in patients.