The test measures the amount of calcium in blood.
There is also a lot of calcium in the bones, but this test only tells you about the amount of calcium in the blood. In urine, it measures the amount of calcium that leaves the body through the kidneys.
The amount of calcium in the blood is maintained by parathyroid hormone (PTH) and vitamin D. When the calcium is too low, the production of PTH increases, releasing calcium from the bones. The kidneys start producing vitamin D, which in turn makes calcium more easily absorbed from the intestines.
If the calcium in the blood is in danger of becoming too high, the production of PTH is reduced, resulting in a balance in the production and requirement of calcium in the blood.