What is the formula to estimate corrected calcium?

• A. Calcium - 0.8(4 - albumin)
• B. Calcium + 0.2(albumin - 4)
• C. Calcium x 1.1
• D. Calcium + 0.8(4 - albumin)

Answer: (D)

Corrected calcium adjusts the measured total calcium for hypoalbuminemia because calcium binds to albumin. When albumin is low, the total calcium can look lower than the true physiologically active level, even though the ionized calcium may be okay. To estimate what the total calcium would be if albumin were normal, you add 0.8 mg/dL for every 1 g/dL that albumin falls below 4 g/dL. So the formula is: corrected calcium = measured total calcium + 0.8 × (4 − albumin in g/dL).

For example, if the measured calcium is 9.0 mg/dL and albumin is 3.0 g/dL, the corrected calcium would be 9.0 + 0.8 × (4 − 3) = 9.8 mg/dL. The other options either subtract instead of add, use an inappropriate factor, or ignore the albumin adjustment entirely, which is why they aren’t correct for estimating corrected calcium.