What is the best diagnostic test for assessing vitamin D deficiency?

The most effective diagnostic test for assessing vitamin D deficiency is the 25-hydroxyvitamin D test. This test measures the level of 25-hydroxyvitamin D, which is the primary form of vitamin D circulating in the bloodstream and serves as an indicator of the overall vitamin D status in the body.

Unlike 1,25-dihydroxyvitamin D, which is the active form of vitamin D and can be influenced by various factors such as kidney function and parathyroid hormone levels, the 25-hydroxyvitamin D concentration reflects both dietary intake and sun exposure over time, making it the most reliable marker for determining vitamin D deficiency.

Calcium level can be affected by many physiological conditions and is not a direct measure of vitamin D status, while parathyroid hormone levels can be elevated in cases of vitamin D deficiency but do not directly indicate vitamin D levels themselves. Therefore, measuring 25-hydroxyvitamin D is the best diagnostic approach for identifying vitamin D deficiency.