What is “ionized calcium”?

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Abstract
“Ionized calcium”, in clinical chemistry, designates the free calcium ions which are present in the biological fluids as hydrated ions: Ca2+, 12 H2O. The calcium ion selective electrode measures differences in activity, however, values are reported not as the activity, but as the activity divided by the activity coefficient (γ) of Ca2+ in “normal plasma”. The latter is assumed to be equal to the activity coefficient (γ) in a calibration solution of CaCl2+ NaCl with an ionic strength of 0,16 mol kg−1. We have estimated the activity coefficient in this calibration solution to be γCa2+ = 0,3040 (molal basis) or γCa2+ = 0,3048 (substance concentration basis). The derivation of γCa2+ is based on a convention for derivation of the activity coefficient of the chloride ion. We have used the Debye-Hückel equation extended by the Stokes-Robinson hydration theory plus the Bates-Staples-Robinson convention that the hydration number for Cl− is zero. International consensus is needed concerning the convention to be used for deriving the activity coefficient of Ca2+ in the calibration solutions and concerning the most appropriate value for γCa2+ in normal plasma.