Vitamin D Blood Test - More Information

The vitamin D test, also known as a vitamin D blood test, vitamin D lab test, D3 blood test, and a 25-hydroxy vitamin D test, measures vitamin D blood levels. Through Accesa Labs, people can order a vitamin D test online, get tested at a local lab, and get results electronically, typically in a few business days.

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What Is a Vitamin D Blood Test?

The vitamin D blood test, also known as the 25-hydroxy vitamin D test or 25-OH vitamin D test, measures the total level of vitamin D in the blood. This vitamin D test measures both vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) and is the most accurate way to assess overall vitamin D status. This test is recommended by the Endocrine Society and other clinical guidelines for evaluating vitamin D deficiency.

More specifically, this vitamin D test measures 25-hydroxyvitamin D [25(OH)D], the storage form of vitamin D that circulates in the bloodstream. This form has a half-life of approximately 2–3 weeks and reflects the body's total vitamin D stores more accurately than the active hormonal form (1,25-dihydroxyvitamin D). This blood test uses a methodology known as LC/MS/MS which is considered the gold standard for 25-OH vitamin D measurement as it can quantify D2 and D3 separately and in a combined total.

Why Is Vitamin D Important?

Vitamin D refers to a class of steroid hormones that have a number of effects in the body. In humans, vitamin D2, also known as ergocalciferol, and vitamin D3, also known as cholecalciferol, are the most important. Vitamin D is also available from foods such as fish and eggs and can also be made into supplements. Additionally, vitamin D can also be made in the skin with the help of UVB radiation from sun exposure. Vitamin D is metabolically activated in the liver and kidney.

Vitamin D - sometimes called the "sunshine vitamin" because it can be produced in the skin from sun exposure - is important for enabling the absorption of nutrients such as phosphate and calcium. Traditionally, certain types of people are thought to be at an increased risk for having low vitamin D levels. More specifically, people who are elderly or obese, people who have had gastrointestinal surgery, babies and people who live in areas with minimal sunlight have a higher likelihood of having low body stores. Because vitamin D plays such an important role in calcium absorption, vitamin D deficiencies can lead to bone weakness and malformation, particularly in the young and the elderly and vitamin D testing is sometimes done concurrently with calcium testing. Some research suggests that Vitamin D may also be involved with reducing cancer risk.

More recent research has demonstrated that Vitamin D might play more important roles in maintaining health than previously imagined. A low vitamin D test level might be associated with an increased risk of a heart attack, cancer, respiratory disorders and immune system dysfunction.

Who Is at Risk for Low Vitamin D?

Certain groups of people are at higher risk of vitamin D deficiency and are more likely to benefit from periodic testing:

People with limited sun exposure. Sunlight is the primary natural source of vitamin D. People who spend most of their time indoors, live in northern latitudes, consistently use sunscreen, or avoid the sun for other reasons produce less vitamin D in the skin.

Older adults. The skin's capacity to synthesize vitamin D can decline with age. Additionally, older adults often have less sun exposure and reduced dietary intake of vitamin D.

People who are overweight. Vitamin D is fat-soluble and can become trapped in fat cells, reducing its availability in the bloodstream. Higher body weight is consistently associated with lower circulating vitamin D levels.

People with gastrointestinal conditions. People with Crohn's disease, celiac disease and short bowel syndrome or who have had gastric bypass surgery in the past can have impaired vitamin D absorption from food and supplements.

People taking certain medications. Some medications, including glucocorticoids (such as prednisone), anticonvulsants, antifungals, and certain cholesterol-lowering drugs, can interfere with vitamin D metabolism or absorption.

Pregnant and breastfeeding women. Vitamin D requirements increase during pregnancy, making maternal vitamin D status an important prenatal concern.

People living in winter or cloudy climates. People living in areas with reduced UVB radiation may have lower Vitamin D production and levels.

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Symptoms of Vitamin D Deficiency

Vitamin D deficiency is among the most common nutritional disorders globally, affecting an estimated one billion individuals across a wide range of ages, ethnicities, and geographic regions. Many people with low vitamin D levels have no obvious symptoms - particularly when deficiency is mild or moderate - which is one reason routine testing is valuable. When symptoms do occur, they often include:

• Persistent fatigue
• Bone pain
• Muscle weakness or cramps
• Frequent illness
• Mood changes
• Hair loss
• Slow wound healing
• Joint pain

Because these symptoms can be found in many different conditions, a vitamin D blood test is the only reliable way to determine whether low vitamin D is a contributing factor. Many people order a vitamin D test specifically because they are experiencing unexplained fatigue or recurrent illness and want to rule out deficiency.

Getting A Vitamin D Test

This vitamin D test is a great measure of a body's vitamin D status. Getting a vitamin D test near you at a Quest Diagnostics lab is a relatively straightforward process. Once you order online and select your lab location, you can go the lab and have your blood drawn for the test. Every standard lab location has the necessary equipment to process a vitamin D blood test and lab results are available electronically relatively quickly. When reviewing your vitamin D lab results, the concentrations of your vitamin D 25-OH total, D2 and D3 levels will be reported in the results column. You can compare those levels to the recommended reference ranges provided by the lab to see where your vitamin D level falls. Because vitamin D can vary from person to person, it is also worth considering getting a vitamin D blood test on a regular basis to track your specific levels over time.

To check additional vitamin levels while getting a vitamin D blood test, check out the advanced vitamin panel or the comprehensive vitamin & mineral panel.

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Interpreting Vitamin D Test Results

The vitamin D blood test measures both D2 and D3 separately and reports them as a combined total. Understanding the difference between the two forms helps interpret what the test is measuring.

Vitamin D3 (cholecalciferol) is produced naturally in the skin when exposed to UVB radiation from sunlight. It is also found in animal-based foods such as fatty fish, egg yolks, and liver and is the form used in most over-the-counter vitamin D supplements. Research consistently shows that vitamin D3 raises serum 25-OH vitamin D levels more effectively and sustains them for longer than an equivalent dose of D2.

Vitamin D2 (ergocalciferol) comes primarily from plant-based food sources and UV-treated mushrooms. It is also the form used in most high-dose prescription vitamin D products. D2 generally raises vitamin D levels less efficiently than D3 and has a shorter half-life once converted.

For most people, the combined 25-OH total is the number that matters for clinical interpretation. Patients who are taking prescription-strength D2 supplements and monitoring their levels over time may find the separately reported D2 and D3 values useful for tracking their response to vitamin D treatment.

Vitamin D - Lab Levels

*Table adapted from StatPearls

While the Endocrine Society generally does not endorse specific 25(OH)D levels to define sufficiency, insufficiency, and deficiency, the clinical measurement of vitamin D levels remains the subject of extensive research. Most clinicians and labs continue to use the 20 ng/mL and 30 ng/mL thresholds as practical reference points for clinical decision-making. Patients with a low or borderline result should discuss supplementation options and appropriate follow-up testing with their healthcare provider. A follow-up vitamin D test after 8–12 weeks of supplementation is often used to confirm that levels have responded to treatment.

Vitamin D Testing for Supplement Monitoring

Many people order a vitamin D blood test not because they have symptoms, but to track the effect of vitamin D supplements they are already taking. Vitamin D supplementation is common and many adults take doses ranging from 1,000 to 5,000 IU daily. However, the optimal supplementation dose varies significantly from person to person depending on baseline levels, body weight, sun exposure, and absorption. A vitamin D blood test is the only reliable way to confirm that supplementation is actually raising levels and that they are in the appropriate range.

Common reasons people test while supplementing with vitamin D include:

• Confirming that a recently started supplement is working
• Adjusting supplement dose based on a blood level
• Ensuring that vitamin D levels are not too high

Patients who test regularly to monitor their vitamin D levels over time may find that annual testing gives the most useful comparison from year to year. Often, vitamin testing is done in the late winter when levels are at their seasonal low.