Vitamin D Testing Guide
A single serum 25-hydroxyvitamin D measurement resolves most vitamin D questions faster than any dietary calculator. Here's what to ask for, how to prepare, and how to interpret the result.
Which test to order
Ask for 25-hydroxyvitamin D  sometimes written as 25(OH)D, calcidiol, or vitamin D, 25-OH total. This is the correct test for assessing vitamin D status: it reflects several weeks of cumulative intake plus cutaneous synthesis, and it's what all the clinical thresholds refer to.
Do not order 1,25-dihydroxyvitamin D (calcitriol) for routine screening. That's the active hormone form, tightly regulated by PTH, and it does not reflect stores. It's ordered only for specific clinical situations  sarcoidosis, chronic kidney disease, hypercalcaemia workup.
How to prepare
- No fasting required. Unlike lipid panels, 25(OH)D is stable in serum and unaffected by recent meals.
- Time of day doesn't matter. 25(OH)D shows minimal circadian variation.
- Don't stop your supplement before testing  the goal is to measure your actual status on your current regimen.
- Do note biotin intake. High-dose biotin (> 5 mg/day) can interfere with some immunoassay methods for 25(OH)D. Pause for 48 hours if you take a hair-and-nails supplement.
Cost and access
- Physician-ordered: covered by most insurance plans for individuals with risk factors (bone disease, malabsorption, chronic corticosteroid use, obesity, dark skin at high latitude).
- Direct-to-consumer: ~US$40–75 without physician order (Quest, LabCorp direct, Everlywell, LetsGetChecked). Results usually within a week.
- At-home fingerstick: ~US$50–75. Slightly less precise than venous draw but adequate for status screening.
When to re-test after starting supplementation
Three months after starting or changing a dose. This is roughly one and a half half-lives of 25(OH)D  long enough for the trajectory to be clear, short enough to catch under- or over-shooting the target early.
Once you're at a stable target level on a stable regimen, annual re-testing is sufficient for most people. Consider testing again after weight change of > 10%, a new medication that affects vitamin D metabolism (glucocorticoids, anticonvulsants), or a season change if you rely on summer sun exposure.
Interpreting the result
Use our blood test interpreter to convert units (ng/mL ↆnmol/L) and see your result against IOM and Endocrine Society thresholds with plain-language interpretation.