In normal serum, what percentage of calcium is ionized?

• A. 50%
• B. 40%
• C. 60%
• D. 70%

Answer: (A)

Calcium in serum exists in three forms: the free, ionized Ca2+ that is biologically active, calcium bound to proteins (mostly albumin), and calcium complexed with small anions like phosphate or citrate. In a normal serum, about half of the total calcium is in the ionized form, roughly 50%. The remaining calcium is divided roughly into 40% bound to albumin and about 10% as complexes with anions. This distribution makes 50% the best estimate for the ionized fraction. Keep in mind that changes in pH or albumin levels can shift this balance, so the ionized fraction may vary even if the total calcium stays the same.