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

Why Is This Test Done?

Your healthcare provider may order this test if you have symptoms of vitamin D deficiency, such as muscle weakness, bone pain, or frequent illnesses. It’s also used to monitor vitamin D levels in people receiving vitamin D supplements or those at higher risk of deficiency, like those with limited sun exposure or certain medical conditions.

What to Expect Before the Test

No special preparation is needed for this blood test. You may be asked to fast for 8–12 hours before the draw if your lab uses a specific method for measuring vitamin D, but this is rare. Bring any recent supplements or medications you’re taking to your appointment.

What Happens During the Test?

A healthcare professional will draw blood from a vein in your arm. The sample is sent to a lab, where it’s analyzed to measure your vitamin D levels, typically using an immunoassay method.

What Do Results Mean?

Results are compared to general reference ranges, but your provider will interpret them in the context of your health history and symptoms. Very low levels may require supplements or dietary changes, while very high levels could indicate overuse of supplements.