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

Why Is This Test Done?

Your healthcare provider may order this test to check for vitamin D deficiency, which can lead to weakened bones (osteoporosis), muscle weakness, or other health problems. It’s often recommended for people with limited sun exposure, certain medical conditions, or those taking medications that affect vitamin D absorption.

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 serum is collected. If plasma is used, fasting may not be required. Bring any medications or supplements you’re taking, as they could affect results.

What Happens During the Test?

A healthcare professional will draw blood from a vein in your arm. The sample is then sent to a lab for analysis using an immunoassay or similar method to measure vitamin D levels.

What the Results Mean

Results are typically reported as a concentration of vitamin D in the blood. Your provider will interpret the results in relation to your health history and symptoms. Very low levels may require supplements or dietary changes, while very high levels may need further investigation.