While vitamin D is still in the bloodstream, before converting to its active form in the kidneys, it is called 25-Hydroxyvitamin D, or 25OHD, according to Quest Diagnostics and MedlinePlus. A blood test measures 25OHD levels. The range of about 30.0 to 74.0 nanograms per milliliter is considered normal.
Testing a blood sample to measure a patient's 25-Hydroxyvitamin D level aids in diagnosing whether a patient has an excess or deficiency of vitamin D, whether a patient's intestines are properly absorbing the vitamin, and how a patient is responding to treatment for a vitamin D-related disorder, according to Quest Diagnostics. In its active form, Vitamin D assists in controlling the body's phosphate and calcium levels, notes MedlinePlus.