If you are experiencing weakness in your bones you should get this test performed. A deficiency or excess of Vitamin D can be the cause of bone weakness, bone malformation, or abnormal metabolism of calcium. Vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat. Because of this, the test can also be used to monitor people with conditions that interfere with fat absorption such as cystic fibrosis and Crohn's Disease.