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 ↔ nmol/L) and see your result against IOM and Endocrine Society thresholds with plain-language interpretation.