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 like fatigue, muscle weakness, or bone pain. It’s also used to monitor vitamin D levels in people with conditions like osteoporosis, kidney disease, or those who don’t get enough sunlight. Some doctors check it during pregnancy or if you’re taking certain medications that affect vitamin D absorption.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the blood draw if your lab uses a specific method. Wear a loose shirt for easy access. Bring any medications or supplements you take daily, as they may affect results.

What Happens During the Test?

A healthcare professional will draw blood from a vein in your arm. You may feel a slight pinch, but it usually doesn’t cause significant discomfort. The blood is sent to a lab for testing, where it’s analyzed for vitamin D levels.

What Do the Results Mean?

Results are typically reported as a numerical value (in ng/mL or nmol/L) compared to a reference range. While reference ranges can vary by lab, low levels may suggest deficiency, while high levels may indicate overabsorption or certain conditions. Your provider will interpret results in the context of your health history.