What percentage of total serum calcium is nondiffusible protein bound?

• A. 80% - 90%
• B. 51% - 60%
• C. 40% - 50%
• D. 10% - 30 %

Answer: (C)

Calcium in serum exists in three forms: ionized (free) calcium, protein-bound calcium, and calcium complexed with anions. The nondiffusible, protein-bound portion is the fraction attached mainly to albumin and other proteins, and it accounts for about 40–50% of the total calcium. This portion does not diffuse readily into tissues, so it’s not available as free calcium and it shifts with changes in serum protein levels. The ionized portion is roughly 45% and is the physiologically active form, while a smaller portion (about 5–10%) is bound to anions. Clinically, fluctuations in albumin can alter total calcium without changing ionized calcium, which is why corrected calcium (to account for albumin) or direct measurement of ionized calcium is used in patients with abnormal albumin.