An ionized calcium test measures only the “free calcium” in your blood that isn’t attached to proteins. An ionized calcium test is more difficult to do, so it’s usually ordered if the results of a total calcium test aren’t normal.

The ionized calcium present in blood plasma is a co-factor for two of the enzymes (blood-clotting factors) involved in the production of fibrin by the clotting cascade, so that calcium is essential for the process of blood coagulation.