How does hypoalbuminemia affect calcium measurements, and how should calcium be reported when albumin is low?

• A. Low albumin lowers total calcium; ionized calcium remains normal; report adjusted calcium using Ca = measured Ca + 0.8 × (4 − albumin)
• B. High albumin lowers calcium
• C. Calcium not affected by albumin
• D. Report calcium as ionized only

Answer: (A)

Calcium in serum exists in two forms: a bound portion attached to albumin and a free, ionized portion that is physiologically active. When albumin is low, the bound calcium portion decreases, so the total calcium measured in the blood falls, even though the ionized calcium—the portion that matters for most body functions—remains normal.

Because total calcium is affected by albumin, clinicians use a correction to estimate the true physiologic calcium when albumin is reduced. The commonly used adjustment adds 0.8 mg/dL to the measured total calcium for every 1 g/dL decrease in albumin from the normal value of 4 g/dL: Ca_adjusted = Ca_measured + 0.8 × (4 − albumin). This gives a value that better reflects the amount of calcium available to tissues.

Ionized calcium is not influenced by albumin and can be measured directly for a precise assessment, but the standard reporting in many labs combines the measured total calcium with an albumin-adjusted value to inform interpretation. High albumin would not cause lower calcium, calcium is indeed affected by albumin binding, and reporting only ionized calcium ignores the routine clinical practice of providing a corrected total calcium when albumin is abnormal.