An isotonic potassium chloride liquid junction minimises the effects of ionic strength, protein and haematocrit on ionised calcium measurement.

When the reference electrode liquid junction of a Nova 2 analyser was changed to isotonic potassium chloride, increasing the ionic strength of aqueous solutions containing a constant total calcium concentration had a negligible effect on measured ionised calcium. In contrast, measurements using hypertonic potassium chloride, hypertonic sodium formate and isotonic sodium chloride liquid junctions showed significant sample ionic strength effects. Interferences by sample protein concentration and haematocrit were marked with hypertonic, but negligible with isotonic junctions. Ionised calcium values in samples containing 25 mmol/L acetate, bicarbonate, beta-hydroxybutyrate, lactate or pyruvate were all lower by 6-7% with an isotonic than a hypertonic potassium chloride junction. Thus, anions that replace bicarbonate during metabolic acidosis have a similar effect on residual liquid junction potential. The clinical usefulness of an isotonic potassium chloride liquid junction needs to be evaluated.