Vitamin D Test Guide: What 25-OH Vitamin D Measures
Learn why 25-hydroxy vitamin D is commonly measured and why the result should be considered together with clinical context.
What does the vitamin D test measure?
The most commonly used blood test for vitamin D status measures 25-hydroxy vitamin D, often written as 25-OH vitamin D. This form reflects vitamin D obtained from sunlight, food and supplements over time.
Why might a doctor request it?
A clinician may request vitamin D testing when there are risk factors for low vitamin D, concerns involving bone health, certain symptoms, or as part of evaluation of another condition. Testing is not required for every healthy person, and the need for it depends on individual circumstances.
Before the test
The test uses a blood sample. Fasting is generally not required for vitamin D alone, but a combined test panel may have different preparation instructions. Tell your clinician about supplements and medicines you take so the result can be interpreted appropriately.
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