Vitamin D Total Concentration (Serum or Plasma)

Why Is This Test Done?

Your healthcare provider may order this test if you have symptoms like fatigue, muscle weakness, bone pain, or if you’re at higher risk for deficiency—such as not getting enough sunlight, having limited sun exposure, or having certain medical conditions like obesity or malabsorption disorders. It may also be checked during routine health evaluations or if you’re taking supplements that could affect vitamin D levels.

What to Expect Before the Test

No special preparation is usually needed. You may be asked to fast before the test, but this depends on your provider’s instructions. Bring any medications you’re taking, as some (like certain anticonvulsants) can lower vitamin D levels.

How the Test Works

A blood sample is drawn from a vein in your arm. The blood is then sent to a lab where it’s analyzed to measure the total vitamin D (both active and inactive forms combined). The method used may include immunoassay or other sensitive techniques to detect the vitamin.

What the Results Mean

Results are typically reported in nanograms per deciliter (ng/dL) or picomoles per liter (pmol/L). While reference ranges vary by lab, very low levels may suggest deficiency, while very high levels could indicate over-supplementation or other conditions. Your provider will interpret results in the context of your health and symptoms.