Serum or plasma mass concentration of vitamin D (25-hydroxyvitamin D)

Why Is This Test Done?

Your healthcare provider may order this test if you have symptoms like muscle weakness, frequent bone fractures, or if you’re at higher risk for vitamin D deficiency—such as not getting enough sunlight, having limited sun exposure, or having certain medical conditions (e.g., malabsorption disorders). It’s also used to monitor vitamin D levels if you’re taking supplements or have a known deficiency.

What to Expect Before the Test

No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the test if it’s a serum sample. Bring any medications or supplements you’re taking, as some (like calcium or vitamin D supplements) could affect results.

How the Test Works

A blood sample is drawn from a vein in your arm. The blood is sent to a lab, where it’s analyzed to measure the total amount of vitamin D (specifically 25-hydroxyvitamin D), which reflects your overall vitamin D status.

What the Results Mean

Results are typically reported in nanograms per milliliter (ng/mL) or picograms per milliliter (pg/mL). Your provider will interpret the results in relation to your health and symptoms. Very low levels may require supplements or lifestyle changes, while higher levels may need monitoring to avoid excess intake.