Free ionized calcium represents approximately what percent of total serum calcium?

Free ionized calcium represents approximately what percent of total serum calcium?

• A. 10
• B. 40
• C. 50
• D. 90

Answer: (C)

Ionized calcium is the physiologically active form of calcium in the blood and makes up about half of the total serum calcium. In a typical serum sample, roughly 50% is free as Ca2+, about 40% is bound to albumin and other proteins, and about 10% is complexed with anions such as phosphate or citrate. Because the ionized portion is the active fraction, the statement that free ionized calcium represents about 50% of total calcium best reflects the usual distribution. Changes in pH or albumin levels can shift this balance, affecting the ionized fraction without drastically altering total calcium.

Ionized calcium is the physiologically active form of calcium in the blood and makes up about half of the total serum calcium. In a typical serum sample, roughly 50% is free as Ca2+, about 40% is bound to albumin and other proteins, and about 10% is complexed with anions such as phosphate or citrate. Because the ionized portion is the active fraction, the statement that free ionized calcium represents about 50% of total calcium best reflects the usual distribution. Changes in pH or albumin levels can shift this balance, affecting the ionized fraction without drastically altering total calcium.