Vitamin D Lab Tests: Impacts and Key Answers on Diagnostics

Reviewed and edited by: Elizabeth Fisher, DCLS, MHA, CC(NRCC), MLS(ASCP)CM, PBTCM

Vitamin D testing has increased rapidly in the last several years. (1) The majority of testing is for screening patients for possible low vitamin D levels in their blood. Has this increased screening testing improved health benefits? Here are answers to some common lab test questions about vitamin D.

What is vitamin D?

Vitamin D is a steroid hormone, also known as calciferol, which aids in calcium absorption in the gastrointestinal tract. Vitamin D helps bone growth and repair by playing a role in maintaining the balance between serum calcium and phosphorus levels. Generally speaking, in the bones, calcium provides strength while phosphorus provides resilience. Vitamin D has also been found to be important for cardiovascular and kidney health.

How does the body obtain vitamin D?

There are three common ways: sunlight, foods, and supplements. When the sun strikes the skin, ultraviolet rays (UV) cause the conversion of cholesterol to vitamin D.

What happens to vitamin D once it is in the body?

Vitamin D formed from sun exposure or obtained from foods is inactive and must be activated. The body does this in two steps. The first step occurs in the liver where vitamin D is converted to 25-hydroxyvitamin D. Then the kidneys convert 25-OH Vitamin D to the biologically active form, 1,25-hydroxyvitamin D25-(OH)2 Vitamin vitamin D. It is this form that aids in the absorption of calcium and phosphorus in the gastrointestinal tract. (2)

What form of vitamin D does the clinical laboratory measure?

The clinical laboratory measures 25-OH Vitamin D. The concentration of 1,25-(OH)2 Vitamin D is 1,000 times lower than 25-hydroxyvitamin D and has a half-life of 4 hours. 25-OH Vitamin D is stable in the blood for up to 2-3 weeks so it is the metabolite that is the most reliable clinical indicator of vitamin D status in lab test reports. (3)

How should you prepare for the test?

No fasting is required. A blood specimen will be taken from a vein in your arm.

What do the results mean?

Measuring 25 OH vitamin D the generally accepted values are (4)

Currently these thresholds may differ slightly depending on the source of the information or the test method. The information below will explain further.

Vitamin D testing challenges faced by the clinical laboratory In the clinical laboratory, the common test method used is an immunoassay test. The test is processed on large instrumentation for lab test reports. There are several different manufacturers with their own unique vitamin D immunoassay method. There is a lack of standardization between the methods. This leads to slight variations in lab test results between the methods. (5)

Are vitamin D screening and vitamin D supplements recommended?

The US Preventive Services Task Force conducted a review in 2021 and concluded that there were no studies that found vitamin D screening improved health outcomes (6) A large randomly controlled trial, Vitamin D and Omega-s Trial (Vital) found vitamin D supplementation did not reduce risk of cancer, cardiovascular disease, fractures, or improve other health outcomes, even among individuals with low baseline 25(OH)D levels (<20 ng/mL) in lab test results. (7)

Is there evidence of an optimal level of 25-OH vitamin D levels?

There is no clear evidence of an optimal level of 25-hydroxyvitamin D25 OH Vitamin vitamin D to prevent disease. Additional research is needed to identify optimal levels for specific health benefits. (8)

Final takeaway: What is the current recommendation for vitamin D testing?

Testing should be reserved for patients with clinical indications such as: hypocalcemia, osteoporosis, malabsorption syndromes, chronic kidney disease, chronic liver disease, and/or symptoms suggestive of osteomalacia (weakening of bones). Moreover, experimental vitamin D supplementation without prior testing is supported only for certain high-risk groups, such as pregnant women, adults who are at least 75 years old, children and adolescents, and individuals with high-risk prediabetes, and those with limited sun exposure (9,10).

Questions to ask your provider

What are the symptoms of vitamin D deficiency?
What is the recommended dietary allowance of vitamin D?
Do I have risk factors for osteoporosis?
Are my calcium levels low due to vitamin D deficiency?
If my calcium levels are low, what other conditions could be causing this?

What role may the clinical laboratory play in helping to ensure appropriate testing?

The clinical laboratory should work with the clinical teams to aid in targeting vitamin D testing. Possible strategies include reviewing test order patterns, working with physician services to add alerts to electronic health records when vitamin D testing in lab test reports is outside guideline recommendations, or consider encouraging physicians to use the Choosing Wisely® program. This program encourages open dialogue between physicians and patients on health outcomes. (11)

Diagnostic Equity Resources

Staring at your lab test report with lab test questions? Understanding your lab test results starts here.