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Vitamin D (25-hydroxy) - Serum or Plasma

Published: August 9, 2026Updated: August 16, 2026

This test measures your levels of vitamin D, a crucial nutrient that helps your body absorb calcium and maintain strong bones. Low levels may indicate deficiency, which can affect bone health and other bodily functions.

Quick summary

["Check your vitamin D levels to ensure proper bone health and overall well-being."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
20946-0
Short code
Vit A Liver-mCnt
Synonyms
Vit A Liver-mCnt
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why Is This Test Done?

Your healthcare provider may order this test if you have symptoms like fatigue, muscle pain, or bone weakness, or if you’re at higher risk for deficiency (e.g., limited sun exposure, certain medical conditions, or dietary habits). It’s also part of routine check-ups for some populations.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the blood draw if your test includes a fasting component (though this is not always required). Bring any medications you’re currently taking.

What Happens During the Test?

A healthcare professional will draw blood from a vein in your arm. The process is quick and usually painless. The sample is then sent to a lab for analysis.

What Are the Results?

Results are typically reported in nanograms per milliliter (ng/mL) or picomoles per liter (pmol/L). Your provider will interpret the results in relation to your health and symptoms.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Low vitamin D levels may increase the risk of bone diseases (like osteoporosis), muscle weakness, or other health issues. High levels can sometimes indicate over-supplementation or certain medical conditions. Always discuss results with your doctor.

Short name

Vitamin D Level

Frequently asked questions

What causes vitamin D deficiency?
Vitamin D deficiency can result from limited sun exposure, poor diet (low intake of fatty fish, fortified foods, or supplements), certain medical conditions (e.g., kidney or liver disease), or medications that interfere with its absorption.
How can I improve my vitamin D levels?
Increase sun exposure (10–30 minutes of midday sun on arms/face 2–3 times a week), eat vitamin D-rich foods (salmon, sardines, egg yolks), or take supplements if recommended by your doctor. Always follow their guidance.
Is vitamin D testing the same as calcium testing?
No, vitamin D testing measures the active form of vitamin D in your blood (25-hydroxyvitamin D), while calcium tests measure calcium levels in your blood or urine. Both are important for bone health but serve different roles.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy