An analysis of measured and calculated calcium quantities in serum

Potentiometrically measured ionized calcium (Ca2+) was compared with total and albumin corrected S-Calcium and calculated Ca2+ in serum samples from blood donors and patients, for whom a request of S-Calcium has been made. Total S-Calcium, Ca2+ and S-Albumin in donors conformed with a simple mass law equation with a mean KCaAlb = 95 l/mol, whereas apparent albumin binding affinity rose with decreasing S-Albumin in patients (due to rise of other calcium binding proteins) with resulting overestimation of albumin corrected and particularly of calculated Ca2+. The frequency and severity of hypoalbuminaemia in patients make calculated Ca2+ unreliable and albumin corrected S-Calcium the clinically most satisfactory routinely practicable substitute for direct Ca2+ measurement.