What It Is
A vitamin D test is a blood test that measures the level of vitamin D circulating in the bloodstream, reflecting the body's overall vitamin D status.
What It Checks
The test typically measures a specific circulating form of vitamin D, which serves as the standard marker used to assess how much vitamin D is available in the body, drawing on the combined contribution of sunlight exposure, diet, and any supplementation.
Why It's Used
Vitamin D plays a role in several bodily processes, including supporting calcium absorption and bone health, which is why the test is commonly used as part of general health evaluations, particularly for people whose sunlight exposure, diet, or other factors might affect vitamin D levels.
How It's Done
A vitamin D test requires a standard blood sample, typically drawn from a vein in the arm, similar to most other routine blood tests. No special preparation, such as fasting, is generally required beforehand.
Background
Vitamin D was identified in the early 20th century, during research into a bone-related condition that was eventually linked to a dietary and sunlight-related deficiency. Understanding of vitamin D's role in the body expanded considerably over subsequent decades, and reliable laboratory methods for measuring circulating vitamin D levels in blood were developed and refined in the latter half of the 20th century, making it possible to directly assess an individual's vitamin D status through routine blood testing, as is commonly done today.