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 muscle weakness, bone pain, or frequent infections. It’s also used to screen for vitamin D deficiency, especially in people with limited sun exposure, darker skin tones, or certain health conditions (e.g., obesity, malabsorption). Pregnant individuals, older adults, and those with chronic illnesses may also need it regularly.

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 your lab uses a different method. Bring any medications you’re taking, as some (like certain anticonvulsants) can affect vitamin D levels.

What Happens During the Test?

A healthcare professional will draw blood from a vein in your arm. The sample is sent to a lab for analysis, where vitamin D levels are measured using an immunoassay or similar method to detect its presence.

What Are the Results?

Results are typically reported as a concentration (e.g., ng/mL or nmol/L), but your doctor will interpret them based on your health history and symptoms. Low levels may trigger follow-up tests or adjustments to diet/sun exposure.