title: Ion effects in measurement of ionized calcium with a calcium-selective electrode.
abstract: We examined the performance characteristics of an improved electrode for measurement of ionized calcium (Orion Research, Inc.) with respect to interferences by Na+, K+, Li+, Mg2+, and H+, as well as the effect of changes in ionic strength and osmolality. At physiologically observed concentrations, all ions except Na+ insignificantly affected ionized-calcium measurements. The decrease in ionized Ca2+ with increased Na+ concentration is caused by the increase in ionic strength and the associated decrease in Ca2+ activity. Sodium had no significant direct effect in concentrations below 250 mmol/L. A previously published (Clin. Chem. 23: 690, 1977) equation for the Na+ correction is invalid for this improved electrode. Changes in osmotic pressure negligibly affect electrode response. Significant changes in ionic strength, as would be observed in severe hypo- and hypernatremic sera, increase or decrease, respectively, values for ionized calcium by changing Ca2+ activity. With aqueous Ca2+ solutions, changes of less than or equal to 3% were observed for Na+ concentrations that deviate from 140 mmol/L by not more than 20 mmol/L. Because the calcium ion activity is considered to be the physiologically important variable, we suggest that no correlation be made for ionic strength effects.
mesh_terms: D002118:Calcium / Q000097:blood*; D002712:Chlorides / Q000378:metabolism; D004566:Electrodes; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008094:Lithium / Q000378:metabolism; D018021:Lithium Chloride; D008274:Magnesium / Q000378:metabolism; D015636:Magnesium Chloride; D009994:Osmolar Concentration; D011189:Potassium Chloride / Q000378:metabolism; D012965:Sodium Chloride / Q000378:metabolism
chemical_list: D002712:Chlorides; D015636:Magnesium Chloride; D012965:Sodium Chloride; D011189:Potassium Chloride; D008094:Lithium; D018021:Lithium Chloride; D008274:Magnesium; D002118:Calcium