Two recent papers by Payne et a1.’S2 propose that an isotonic salt bridge of either 150 mmol/L KCl or NaCl may lead to improvements in performance over the hypertonic salt bridges currently used by manufacturers of ion-selective electrode based clinical analysers. Primary among their evidence in reaching this conclusion is a small increase in ionized calcium concentration measured with several commercial analysers employing hypertonic salt bridges, which coincides with an albumin peak in a gel filtration experiment. While we cannot argue with these data regarding protein interference, we believe that there are other considerations, which the authors failed to recognize, before a change to an isotonic salt bridge can be proposed. The authors state in the opening paragraph of their first paper:‘... the behaviour of liquid junctions between hypertonic salt bridges and protein-containing solutions is more complex than that predicted by models such as the Henderson equation.’We agree with this statement. However, what the Henderson equation does predict is that the residual liquid junction potential (difference in liquid junction potential between calibrating solution and sample) becomes larger as the concentration of equitransferant salt in the salt bridge is lowered. Thus, the accuracy of a sample measurement deteriorates as the concentration of the salt bridge electrolyte is lowered. The accuracy is further worsened as the ionic compositions of the sample and the calibrating solution begin to differ greatly, ie, in alkalotic patients with high bicarbonate.’In addition to the question of accuracy, there is the issue of agreement among the ion analysers from various manufacturers. Differences in the composition of calibrating solutions between commercial systems will also be reflected in the magnitude of the residual liquid junction potential where an isotonic salt bridge is utilized. The same data presented by Payne el al. in their first paper illustrate this fact. I The ionized calcium analysers of Nova, Radiometer and Ciba-Corning agree closely for the protein-containing fractions (Nos 7-12) in their Fig. 2. However, when the authors replaced the proper salt bridges of these analysers with 150mmol/L KC1, the three analysers differed greatly in their measurements of the same samples, as much as 0.4 mmol/L, in the fractions containing the highest protein concentrations (their Fig. 3 data). The current trend is toward standardization of ion-electrode based analysers from different manufacturers. Recently, international collaborative efforts between manufacturers, clinicians and government have resulted in both materialsJ and methods5 for achieving standardization of analysers for Na+ and K+. Similar efforts are underway for ionized calcium. 6 Any changes to the analytical system which result in a greater disagreement among commercial analysers should be avoided.