Vitamin D (D2, D3) QuestAssureD 25-Hydroxy Test

Vitamin D exists in two main dietary forms. D3, or cholecalciferol, forms in the skin with sunlight and appears in some animal-based foods and supplements. D2, or ergocalciferol, comes largely from plant-derived sources and certain supplements. Both are converted by the liver into 25-hydroxyvitamin D, the circulating form used to gauge status. This QuestAssureD version reports the total along with the individual D2 and D3 contributions, which can give a clinician a more detailed view of where a person's vitamin D is coming from.

The test is done with a simple blood draw at a Quest patient service center. A phlebotomist collects a sample from a vein, usually in the arm, and it is sent to the laboratory. No special preparation is typically required, though people follow any instructions their ordering clinician provides. A clinician reads the result — total and the separate forms — in the context of symptoms, history, and anything else measured alongside it.

People often look into a vitamin D marker when they are exploring bone and muscle health, general fatigue, or aches without an obvious cause, and some prefer a version that breaks out the D2 and D3 forms. Seeing the forms separately can be helpful when a person is taking a supplement and a clinician wants to understand how much each source is contributing. Others simply want to follow their status over time. As with any single marker, it is most useful as one part of a broader picture rather than something read entirely on its own.

The test measures 25-hydroxyvitamin D, the form that best represents how much vitamin D is available from all sources combined. What sets this version apart is that it reports the D2 and D3 forms on their own as well as together, so the breakdown is visible rather than blended into a single number.

That detail can matter because the two forms tend to trace back to different origins — D3 to sunlight and animal-based sources, D2 more often to plant-based sources and specific supplements. A clinician may find the split useful, but the individual forms still need to be interpreted together and in context, not read in isolation.

Reporting the forms separately can help a clinician see the fingerprint of what a person is taking or eating. For example, a noticeable D2 contribution may point toward a particular kind of supplement, while D3 reflects sunlight and animal-based sources. This is descriptive information rather than a verdict on its own. The clinician weighs the pattern alongside a person's habits, symptoms, and any other markers before drawing conclusions about overall status.

No appointment is needed. People can walk into a Quest patient service center during open hours, and the draw itself is quick — a phlebotomist collects a small sample from a vein in the arm, and most visits wrap up in a few minutes. Once the laboratory completes its analysis, the result, including the D2 and D3 breakdown, is posted to the account, where it can be reviewed and shared with a clinician.

Results will be emailed securely to you as a PDF.