Calcium occurs in three forms in our blood.
- Bound to albumin: 45% of total plasma calcium is bound to plasma proteins, predominantly albumin. This calcium is inert or not metabolically active.
- Bound to Citrate or other plasma molecules: About 5% of serum Calcium is complexed with plasma molecules, this calcium is also inert or inactive.
- Ionized Calcium: It is the ionized calcium which is responsible for all the metabolic function of plasma Calcium, About 50% of plasma calcium is in ionized form.
- Normal range of ionized calcium is 4.4-5.4mg/dl
- Normal range of total calcium is 8.9-11mg/dl
What is Corrected Calcium
In case of hypoalbuminemia, corrected Calcium is used instead of normally measured laboratory value of total serum calcium, because of in case of hypoalbuminemia, more metabolically active calcium is available.
For example,
Lets say patient’s Serum Calcium Level is 7.5 mg/dl and patient albumin is 4.0 g/dl. In this case patient’s ionized calcium will also be low as the albumin levels are normal and 45% of this 7.5mg/dl of total serum Calcium will be bound to 4.0g/dl of albumin and only 3.37 mg/dl of serum calcium will be available for metabolic functions which is lower than normal required range, that is 4.4-5.0 mg/d.
But in case of hypoalbuminemia, if total calcium is low, the metabolically active ionized calcium can be normal, because in this case, less calcium is bound to albumin as albumin level is not 4 g/dl.
For example, if patient Calcium is 7.5 mg/dl and albumin is 2.0 g/dl. In case of albumin of 4.0 g/dl, 50% of total serum calcium is protein bound, but in case of 2.0 g/dl albumin less than 50% of total calcium will bound to protein, because protein level is low, and less proteins are available to make a bound with calcium. So in order to correct the total calcium in the presence of hypoalbuminemia following formula is used;
Formula to Calculate Corrected Calcium In Case of Hypoalbuminemia
Corrected Calcium (mg/dl) = 0.8 (4.0 – Pt’s albumin in mg/dl) + Pt’s Calcium
In above example; in which patient’s calcium is 7.5 mg/dl and albumin is 2.0 g/dl the corrected calcium will be;
Corrected Ca++ (mg/dl) = 0.8 (4.0 – 2.0) + 7.5
= 9.1mg/dl
So the Corrected Calcium for albumin is normal. It means patient doesn’t have hypocalcemia and his metabolically active caclium (ionized calcium is normal).
