American Board of Family Medicine (ABFM) Exam 2025 – 400 Free Practice Questions to Pass the Exam

The most effective diagnostic method for assessing vitamin D deficiency is the measurement of 25-hydroxyvitamin D. This compound is the major circulating form of vitamin D in the body and serves as a reliable indicator of overall vitamin D status. When vitamin D is ingested or produced in the skin, it is converted in the liver to 25-hydroxyvitamin D, which is then further metabolized in the kidneys to 1,25-hydroxyvitamin D. However, the 1,25-hydroxyvitamin D form, albeit active, has a short half-life and is influenced by various factors such as calcium levels and parathyroid hormone, making it less reliable for assessing overall vitamin D sufficiency.

Other tests like ionized calcium and serum phosphorus may provide information about calcium metabolism, but they do not specifically quantify vitamin D levels. Hence, they are not suitable as standalone diagnostic methods for vitamin D deficiency. The measurement of 25-hydroxyvitamin D is considered the gold standard, as it accurately reflects recent dietary intake and sun exposure, thereby providing a clear picture of a person's vitamin D status.