Calcium measured in the laboratory corresponds to total calcium, it includes:
50% ionized or free calcium;
10% complexed calcium;
40% protein-bound calcium.
Only the ionized fraction is biologically active.
Hypoalbuminemia or hyperalbuminemia can modify the protein-bound fraction, which respectively causes a decrease or an increase in the total calcemia measured without the ionized fraction being modified. We therefore rely on correction formulas to estimate the fraction of ionized calcium:
Depending on albuminemia: Corrected calcium = Measured serum calcium - 0.025 * (albumin - 40) = Measured calcium + 0.025 * (40 - albumin) with calcium in mmol L-1 and albumin in g L-1 )
Depending on protein: Corrected calcium = Measured calcium / (0.55 + (protein / 160)) (with calcium in mmol L-1 and protein in g L-1).
These formulas tend to underestimate calcium when albumin becomes greater than 40 g/l2. It is therefore also possible to carry out a direct determination of plasma ionized calcium. The dosage of ionized calcium is also used in intensive care, in newborns for the dosage of neonatal hypocalcaemia, in the event of major surgery or infusions.