A vitamin D assay is a blood test that measures the concentration of vitamin D in the body. This diagnostic tool is used to identify a vitamin D deficiency or, in rarer cases, toxicity from excessive levels. The test measures a chemical called 25-hydroxyvitamin D, or 25(OH)D, which is the most accurate indicator of the body’s overall vitamin D stores.
Purpose of Vitamin D Testing
A healthcare provider may order a vitamin D test when a person exhibits symptoms of a deficiency. These signs can be subtle and include bone pain, muscle weakness or aches, and fatigue. In children, a deficiency can lead to rickets, which involves bone malformation, while in adults it can cause osteomalacia, a similar condition of bone softening. The test helps determine if low vitamin D levels contribute to these symptoms or to conditions like osteoporosis.
The test is also used for individuals with known risk factors for deficiency. People with limited sun exposure are at high risk, as are older adults whose skin is less efficient at synthesizing vitamin D. Individuals with darker skin tones have higher amounts of melanin, which also reduces the skin’s ability to produce vitamin D from sunlight.
Certain medical conditions increase the risk of deficiency. Diseases that affect fat absorption, such as Crohn’s or celiac disease, can impair the absorption of this fat-soluble vitamin. Individuals who have undergone gastric bypass surgery may also have difficulty absorbing sufficient nutrients. Chronic kidney and liver diseases are a concern because these organs help convert vitamin D into its usable forms.
The test also serves a monitoring function for individuals diagnosed with a deficiency and undergoing treatment. After a patient starts supplementation, a provider may re-test their levels to ensure the treatment is effective. The goal is to confirm that the person’s vitamin D stores are returning to a sufficient range.
The Vitamin D Testing Process
The vitamin D test is a straightforward procedure involving a simple blood draw. A healthcare professional collects a blood sample from a vein, usually in the arm, which is then sent to a laboratory for analysis.
No special preparations, such as fasting, are required for a standard 25-hydroxyvitamin D test. However, patients should follow any specific instructions from their provider, as other tests may be conducted simultaneously.
The primary substance measured is 25-hydroxyvitamin D, or 25(OH)D. This form is measured because it is the main storage form of vitamin D and has a long half-life of about two to three weeks. This stability makes its blood concentration the most reliable indicator of a person’s overall vitamin D status from both diet and sun exposure.
Another form of vitamin D, 1,25-dihydroxyvitamin D or calcitriol, is the active hormone. This form is not measured when screening for deficiency because its levels can remain normal even when a person is deficient in 25(OH)D. Testing for the active form is reserved for specific situations, like chronic kidney disease, to investigate issues with vitamin D metabolism.
Understanding Vitamin D Test Results
Vitamin D concentration is reported in nanograms per milliliter (ng/mL) or, in some regions, nanomoles per liter (nmol/L). The units on the lab report are necessary for correct interpretation of the values.
Medical organizations have established reference ranges to classify vitamin D status. A level below 20 ng/mL is defined as a vitamin D deficiency. This level is associated with an increased risk for bone-related health issues.
A level between 21 ng/mL and 29 ng/mL is classified as vitamin D insufficiency. While not fully deficient, individuals in this category lack an optimal amount of vitamin D for bone and overall health. A provider might recommend lifestyle changes or supplementation to raise these levels.
The target range for most healthy individuals is between 30 ng/mL and 100 ng/mL, which is defined as sufficient. Levels within this window are adequate for supporting bone health and overall physiological processes. Maintaining levels in this range is the goal of any treatment plan.
Levels above 100 ng/mL are considered high and could indicate potential toxicity, though some experts place the threshold at 150 ng/mL. High concentrations are almost always caused by excessive supplement doses, not diet or sun exposure, and can lead to symptoms like nausea and weakness.
Different Methods of Measurement
Vitamin D tests are performed using different laboratory technologies, which can influence the results. The two main methods are immunoassays and liquid chromatography-tandem mass spectrometry (LC-MS/MS).
Immunoassays are the most widely used method for measuring vitamin D levels. These automated tests are faster and less expensive, working by using antibodies that bind to the 25(OH)D molecule. The test measures this binding to determine the vitamin D concentration in the blood.
Immunoassays can be less accurate because the antibodies may bind to other vitamin D metabolites in the blood, an issue called cross-reactivity. This can lead to a falsely high or low result, potentially affecting clinical decisions for results near a diagnostic cutoff.
Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is considered the gold standard for vitamin D measurement due to its high accuracy. This technique physically separates the 25(OH)D molecule from other substances in the blood before measuring its precise mass.
This separation step eliminates the cross-reactivity issues that affect immunoassays. While LC-MS/MS is more complex and expensive, it provides a more reliable measurement. An immunoassay is sufficient for most routine screening, but a provider might prefer LC-MS/MS for complex cases or to confirm borderline results.