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

Your healthcare provider may order this test if you have signs of low vitamin D, such as bone pain, muscle weakness, or ongoing fatigue, or if you have a condition that makes it hard for your body to absorb nutrients, like celiac disease or Crohn's disease. It may also be used to check bone health, monitor supplement therapy, or evaluate your risk of osteoporosis.

Your healthcare provider may recommend this test if you have symptoms like muscle weakness, bone pain, or if you’re at risk for vitamin D deficiency. It’s also used to monitor vitamin D levels in people taking supplements or those with conditions affecting vitamin D metabolism.

This test is a simple blood draw that measures the amount of 25-hydroxyvitamin D, the main form of vitamin D circulating in your blood. A small sample is taken from a vein, usually in your arm, and sent to a laboratory, where the level is measured and reported in nanograms per milliliter (ng/mL).

A small blood sample is taken, and the vitamin D in your serum or plasma is measured using a specialized lab technique. This helps determine whether your levels are sufficient or if you need to adjust your diet, supplements, or lifestyle.

Most people do not need special preparation for a vitamin D blood test, although your provider may ask you to fast briefly if other blood tests are being done at the same time. Let your healthcare team know about all vitamins, supplements, and medications you take, because some of these can affect your results.

No special preparation is usually needed, but you may be asked to fast for a few hours if your provider specifies. Wear comfortable clothing for the blood draw, and let your provider know about any medications you’re taking.

Your results are reported in nanograms per milliliter (ng/mL). A value within the normal range means your body has enough vitamin D to support healthy bones, muscles, and immune function. Low levels may point to a deficiency that can weaken bones over time, while very high levels, though uncommon, can result from taking too many supplements. Your provider will explain what your specific number means for you.

Results help your healthcare provider assess whether your vitamin D levels are within a healthy range. Low levels may indicate a need for dietary changes, supplements, or further medical evaluation.