Which statement best differentiates iCAL from TCAL?

• A. iCAL is controlled by PTH and changes inversely with pH; TCAL is controlled by albumin and remains constant with pH.
• B. iCAL is controlled by calcitonin and TCAL by parathyroid hormone.
• C. iCAL and TCAL are both controlled by PTH but respond differently to temperature.
• D. iCAL is controlled by albumin and TCAL by PTH.

Answer: (A)

Ionized calcium (iCAL) and total calcium (TCAL) are regulated differently. iCAL is the biologically active form and is tightly influenced by parathyroid hormone and by acid-base status. The pH of blood affects how calcium binds to albumin: in acidosis (low pH) more H+ displaces calcium from albumin, increasing the free ionized calcium; in alkalosis (high pH) less displacement occurs, lowering the ionized calcium. So iCAL changes inversely with pH. TCAL reflects the amount bound to albumin plus the free ionized portion, so it tracks albumin levels rather than pH itself; when albumin changes, TCAL shifts, but pH shifts don’t push TCAL around in the same way. Therefore, the best distinction is that iCAL is controlled by PTH and varies inversely with pH, while TCAL is controlled by albumin and remains relatively constant with pH.