What is the difference between calcium and ionized calcium?

A calcium level is often ordered on routine blood tests. People with hyperparathyroidism or hypoparathyroidism are often monitored with calcium blood results. Patients who have had thyroid or parathyroid surgery need to be monitored with calcium blood results as well. The calcium blood test generally ordered in these situations is serum calcium. Elevated serum calcium means that the total amount of calcium in the blood elevated. Decreased serum calcium means that the total amount of calcium in the blood is decreased. This test usually gives sufficient information to screen patients for disease or to monitor them over time.

Calcium in the blood can be ionized or bound. Bound calcium is bound to proteins in the blood such as albumin. This calcium is not free to go in and out of cells as needed or act in chemical reactions. Ionized calcium is the active calcium in the blood. It is not attached to proteins, rather it floats freely in the blood. This is the calcium that can move in and out of cells to enable actions or reactions such as muscle flection or sending an impulse down a nerve.

In a complicated patient or when the calcium blood results do not show what is expected, an ionized calcium test may be ordered. Because ionized calcium is the active form, ionized calcium levels are kept within a very narrow range. On the other hand, total calcium levels can fluctuate slightly based on the volume of blood and other factors. Therefore, a person who is dehydrated my have a slightly increased total calcium. However, their ionized calcium levels will remain normal.