Ionized calcium blood test measures the total calcium (50% to 55%) in blood. Calcium is a mineral that is responsible for strong and healthy bones, teeth and muscles. Ionized calcium or free calcium is in the most physiologically active form. Persons with parathyroid gland problems, kidney diseases and certain types of cancers are tested for ionized calcium level. Normally, the calcium level in our body is well controlled. There is a balance between the free or ionized calcium present in blood and the stored or bound calcium present in bones. Any imbalance in their level can cause serious health problems. A critically ill person on IV fluids or before receiving blood transfusion may need to undergo ionized calcium blood test. Persons with abnormal calcium values are required to do an ionized calcium retest.