Vitamin D, 25-hydroxy is an inactive form of the fat-soluble vitamin D that is responsible for maintaining good bone growth and healthy tissues. The kidneys metabolize this form of the vitamin into one that is easier for the body to use, called vitamin D, 1, 25-hydroxy, but this form of the vitamin is somewhat difficult to measure. This makes vitamin D, 25-hydroxy tests the most accurate method of measuring how much vitamin D is in a person's bloodstream.

D vitamins are responsible for controlling the body's phosphate and calcium levels, which in turn affect everything from mood to bone growth to a person's likelihood of developing cancer. Long-term deficiencies in vitamin D can result in severe problems, including a higher risk of asthma, permanent physical and mental disabilities and cardiovascular diseases. Most people who eat a balanced diet and spend at least 15 to 20 minutes per day outdoors have adequate levels of vitamin D, but people who have digestion disorders such as Chron disease, irritable bowel syndrome or celiac disease may not be able to absorb the vitamin properly. Incorrect absorption is also a problem for many people who have had gastric bypass surgery.

A vitamin D , 25-hydroxy test can help determine whether a person is eating an adequate diet or getting vitamin D properly through other sources. Most people will have blood levels of this vitamin within the 30 to 74 nanogram per milliliter range.

This test requires FASTING for approximately 10 hours to obtain accurate results.