Which statement about calcium measurement is correct?

Which statement about calcium measurement is correct?

• A. Protein-bound calcium is easily measured directly
• B. Total calcium equals protein-bound calcium plus free calcium
• C. Free calcium is always equal to total calcium
• D. Serum calcium is independent of albumin

Answer: (B)

Calcium in serum exists in three forms: the ionized (free) portion, the protein-bound portion (mostly to albumin), and a small portion that is complexed with anions. In routine lab reporting, the total calcium you see is essentially the sum of the free calcium and the protein-bound calcium (the complexed part is relatively small). So, total calcium ≈ free calcium + protein-bound calcium. This is why this statement is the best choice. This matters clinically because albumin levels influence how much calcium is bound to protein. If albumin is low, total calcium can fall even if the physiologically active ionized calcium is normal. That’s why corrected calcium calculations are used when albumin is abnormal. The ionized calcium fraction is the true active component, and it is not reliably inferred from total calcium when albumin or binding capacity changes. The other options don’t fit because: routine calcium tests don’t measure protein-bound calcium directly as a separate fraction; free calcium is not equal to total calcium since total includes both free and bound portions; and serum calcium does depend on albumin, so it isn’t independent of albumin.

Calcium in serum exists in three forms: the ionized (free) portion, the protein-bound portion (mostly to albumin), and a small portion that is complexed with anions. In routine lab reporting, the total calcium you see is essentially the sum of the free calcium and the protein-bound calcium (the complexed part is relatively small). So, total calcium ≈ free calcium + protein-bound calcium. This is why this statement is the best choice.

This matters clinically because albumin levels influence how much calcium is bound to protein. If albumin is low, total calcium can fall even if the physiologically active ionized calcium is normal. That’s why corrected calcium calculations are used when albumin is abnormal. The ionized calcium fraction is the true active component, and it is not reliably inferred from total calcium when albumin or binding capacity changes.

The other options don’t fit because: routine calcium tests don’t measure protein-bound calcium directly as a separate fraction; free calcium is not equal to total calcium since total includes both free and bound portions; and serum calcium does depend on albumin, so it isn’t independent of albumin.