Serum/plasma mass 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, frequent infections, or if you have conditions like osteoporosis or rickets. It may also be done to check vitamin D levels in people with limited sun exposure, certain dietary habits, or medical conditions that affect vitamin D absorption.

What to Expect Before the Test

No special preparation is usually needed. You may be asked to fast for 8–12 hours before the test if it involves drawing blood, but confirm with your lab or doctor. If you’re taking vitamin D supplements, discuss timing with your provider to avoid affecting results.

What Happens During the Test?

A healthcare professional will draw blood from a vein in your arm. The blood sample is sent to a lab for analysis, where vitamin D levels are measured using a high-performance liquid chromatography (HPLC) method.

What the Results Mean

Results are typically reported in nanograms per milliliter (ng/mL) or picomoles per liter (pmol/L). While reference ranges vary by lab, low levels may suggest deficiency, while higher levels could indicate over-supplementation or other conditions. Your doctor will interpret results in the context of your health history.