What does 'free' calcium refer to?

• A. Calcium bound to albumin
• B. Ionized calcium
• C. Calcium stored in bone
• D. Calcium bound to phosphate

Answer: (B)

Free calcium refers to the ionized form of calcium—the portion that is not bound to proteins like albumin or to other molecules. This ionized calcium is the physiologically active form that directly drives processes such as muscle contraction, nerve signaling, coagulation, and enzyme activity. Total calcium in the blood includes both bound and free forms, so factors like albumin levels and pH can change the amount that is bound and thus total calcium without changing the actual ionized calcium. Calcium stored in bone is a reservoir, not the circulating free fraction, and calcium bound to phosphate is another bound form, not the active free form.