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Vitamin D TestingTest code(s) 17306, 92888, 91935
Question 1. What is vitamin D?
Vitamin D is a fat-soluble vitamin that occurs in 2 forms: vitamin D3 and vitamin D2. Vitamin D3, the more common form, is made in the skin after exposure to sunlight. Vitamin D2, on the other hand, comes mostly from food and over-the-counter supplements. It may also be used in the pharmacological treatment of vitamin D deficiency.
Vitamin D is rapidly metabolized in the liver to 25-hydroxyvitamin D (25[OH]D). This inactive form is then converted in the kidneys to the active 1,25-dihydroxyvitamin D form.
Question 2. What does vitamin D do in the body?
Vitamin D helps maintain healthy levels of calcium and phosphorus by aiding in their absorption from the gut. This helps the body form and maintain strong bones. Vitamin D also modulates neuromuscular, immune, and other cellular functions. Vitamin D deficiency has been associated with a wide range of medical conditions including heart disease, hypertension, diabetes, and cancer.
Question 3. What is the clinical application of vitamin D testing?
The concentration of serum 25-hydroxyvitamin D (25[OH]D) provides a good index of circulating vitamin D stores in patients not suffering from renal disease. Low 25(OH)D concentrations can result from inadequate sunlight, a dietary deficiency, poor absorption of the vitamin, or impaired metabolism of the sterol in the liver. A 25(OH)D deficiency can lead to bone diseases such as rickets and osteomalacia.
Deficiency has also been associated with a wide range of medical conditions including heart disease, hypertension, diabetes, and cancer.
The Endocrine Society recommends screening individuals at risk for deficiency. These include those with1:
- Chronic kidney disease
- Hepatic failure
- Malabsorption syndromes
- Medications (anti-seizure medications, glucocorticoids, AIDS medications, antifungals, cholestyramine)
The Society also recommends screening1:
- African-American and Hispanic children and adults
- Pregnant and lactating women
- Older adults with history of falls
- Older adults with history of nontraumatic fractures
- Obese children and adults
The Endocrine Society also recommends monitoring patients with granuloma-forming disorders and some lymphomas by testing 25(OH)D and serum calcium.1 Some physicians may wish to monitor people receiving vitamin D therapy to evaluate for compliance and expected change in concentration.
Question 5. How much vitamin D do people need?
To maximize bone health, the Endocrine Society suggests a dietary intake of at least 400 IU/day for infants <1 year and at least 600 IU/day forchildren 1 year and older.1 Whether these levels are enough to provide all the nonskeletal benefits of vitamin D is not known. At least 1000 IU/day may be needed to raise the blood level consistently above 30 ng/mL (cut point for vitamin D sufficiency).1
To maximize bone health and muscle function in adults 19 to 70 years of age, the Endocrine Society suggests a dietary intake of at least 600 IU/day.1 Whether these levels are enough to provide all the nonskeletal benefits of vitamin D is not known. At least 1500–2000 IU/day may be needed to achieve a blood level of 30 ng/mL.1
To maximize bone health and muscle function in adults over 70 years, the Endocrine Society suggests a dietary intake of at least 800 IU/day.1 Whether these levels are enough to provide all the nonskeletal benefits of vitamin D is not known. At least 1500–2000 IU/day may be needed to achieve a blood level of 30 ng/mL.1
Obese children and adults and those on certain medications may need at least 2 to 3 times the suggested dietary intake for their age group.1 Relevant medications include anticonvulsants, glucocorticoids, AIDS medications, and antifungals such as ketoconazole.
For people who are vitamin D insufficient or deficient (see Question 14 for definitions), supplementation or a therapeutic prescription may be needed to correct the deficiency. Refer to the Endocrine Society guidelines1 for treatment recommendations.
Question 6. What are the sources of vitamin D?
Vitamin D can be obtained from exposure to sunlight. However, sun exposure can be affected by season of the year, latitude, time of day, skin pigmentation, use of sunscreens, and age. These variables may necessitate alternative sources for some people.
One alternative source is the diet. Some foods are naturally high in vitamin D; these include oil-rich fish such as salmon, mackerel, and herring. For example, fresh farmed salmon may have approximately 100-250 IU in 3.5 ounces, whereas fresh, wild caught salmon may have approximately 600-1000 IU in a 3.5 ounce serving. Shiitake mushrooms, especially sun-dried, are also high in vitamin D. Other foods are fortified with vitamin D; these include milk and other dairy products, orange juice, and some grain products.
Multivitamin and other supplements are another alternative source.
None of these sources may be adequate for people with liver or kidney disease as they may be unable to produce sufficient amounts of the active form of vitamin D. This is because vitamin D metabolism to the active form requires the liver and kidney. These people may need supplementation with the active form (1,25-dihydroxyvitamin D).
Question 7. What is the impact of seasons on vitamin D?
25(OH)D concentrations are typically at their lowest at the end of February and at their highest at the end of August. This seasonal effect is more notable in northern latitudes than in southern latitudes where the sun is out for more of the year. Thus, there may be more of a need to supplement, or to supplement with higher doses of vitamin D, in the winter months than in the summer months.
Quest Diagnostics data show that the percentage of patients who are deficient in vitamin D vary seasonally from 21% at the end of summer and 48% at the end of winter.
Question 8. How common is vitamin D deficiency?
Based on a sample of patients throughout the United States, Quest Diagnostics observed that 33% of patients were deficient in vitamin D, and 60% were either deficient or suboptimal.
Question 9. What are the Endocrine Society guidelines?
The Endocrine Society is a professional organization that focuses on hormone research and areas that impact the clinical practice of endocrinology. The Endocrine Society publishes Clinical Practice Guidelines (CPGs) on endocrine-related topics. The guidelines are developed by experts in the field who rely on evidence-based studies and reviews of published literature.
In 2011, the Endocrine Society issued a Clinical Practice Guideline titled “Evaluation, Treatment, and Prevention of Vitamin D Deficiency”. This was published in the Journal of Clinical Endocrinology and Metabolism, volume 96, pages 1911-1930. This guideline is available at https://www.endocrine.org/~/media/endosociety/Files/Publications/Clinical%20Practice%20Guidelines/FINAL-Standalone-Vitamin-D-Guideline.pdf.
Summary of the guideline1:
“Considering that vitamin D deficiency is very common in all age groups and that few foods contain vitamin D, the Task Force recommended supplementation at suggested daily intake and tolerable upper limit levels, depending on age and clinical circumstances. The Task Force also suggested the measurement of 25-hydroxyvitamin D level by a reliable assay as the initial diagnostic test in patients at risk for deficiency. Treatment with either vitamin D2 or vitamin D3 was recommended for deficient patients. At the present time, there is not sufficient evidence to recommend screening individuals who are not at risk for deficiency or to prescribe vitamin D to attain the noncalcemic benefit for cardiovascular protection.”
Question 10. What does vitamin D testing measure?
Vitamin D tests generally measure the total concentration of 25(OH)D, which is the main form of vitamin D circulating in blood and the best indicator of vitamin D deficiency or excess. Vitamin D tests using liquid chromatography, tandem mass spectrometry (LC/MS/MS) may also provide the concentration of vitamin D2 and D3 which, when added together, equal the total vitamin D concentration. For detection of vitamin D deficiency, measurement of 1,25-dihydroxyvitamin D is not recommended, as levels may be misleadingly normal in patients with significant vitamin D deficiency.
Question 11. Why do physicians test for vitamin D?
A physician generally will order a test to determine the level of vitamin D in a patient's body. A physician would typically evaluate the test result in connection with several other factors affecting a patient's health such as medical history, gender, and age. The Endocrine Society guidelines suggest testing those listed in Question 4.
Question 12. How does Quest Diagnostics test for vitamin D?
Quest Diagnostics offers two methods. Both methods have passed rigorous internal and external validations, and both are considered accurate and reliable. The first method is an immunoassay. It provides high quality quantitative results that are tied back to standards from the National Institute of Standards and Technology (NIST Standard Reference Material [SRM] 972). It was recently certified by the Centers for Disease Control Vitamin D Standardization Certification Program and is suitable for most individuals.
The second method is a liquid chromatography, tandem mass spectrometry method, also known as LC/MS/MS. This method is appropriate for people who have been prescribed vitamin D2 or when separate measurement of vitamin D2 and D3, in addition to total vitamin D, is required.
Infants (birth up to 3 years) may have circulating levels of the 25-hydroxyvitamin D3 3-epimer. This is a low activity form of vitamin D. The immunoassay does not detect the 3-epimer, and results are not influenced by its presence. The LC/MS/MS, however, does detect the 3-epimer and therefore overestimates total vitamin D if the 3-epimer is not accounted for. To address this, Quest Diagnostics offers a separate LC/MS/MS test for infants less than 3 years of age. The test for infants involves separation of the 25(OH)D3 3-epimer and exclusion of it from the 25(OH)D3 and total vitamin D results reported. For more information on the measurement of 3-epimer visit: http://journals.lww.com/jinvestigativemed/Fulltext/2014/04000/The_Measurement_of_3_Epimer_25_Hydroxyvitamin_D_by.7.aspx.
Question 13. Which test should I order?
The immunoassay method offered by Quest Diagnostics provides reliable results that are suitable for most situations. It generally provides quicker turnaround time. The results are typically available the day after specimen collection and submission. Since this is similar to other “routine” tests, vitamin D test results can be reported at the same time as many other tests. Although the immunoassay detects 25(OH)D2 and 25(OH)D3, it does not differentiate them; only the total 25(OH)D concentration is reported.
LC/MS/MS is also a reliable testing method and is considered the reference method. Unlike the immunoassay, the LC/MS/MS differentiates 25(OH)D2 and 25(OH)D3 and reports their concentrations along with the total 25(OH)D concentration. Thus, it is suitable for differentiating the contribution of prescription supplementation from sunlight and over-the-counter supplementation. This is because vitamin D2 is typically used in subscription formulations, vitamin D3 is the form produced subsequent to sun exposure, and vitamin D3 is more often used in over-the-counter supplements.
For infants less than three years of age, either the immunoassay or the LC/MS/MS test code that is specifically for infants (test code 91935) is suitable.
All of our 25(OH)D tests provide the concentration of total 25(OH)D in a patient's serum, the total 25(OH)D reference range, and suggested cut points to define optimal, insufficiency, and deficiency vitamin D status. The latter are based on the Endocrine Society guideline:
<20 ng/mL Deficiency
20-29 ng/mL Insufficiency
≥30 ng/mL Optimal
In addition, tests using the LC/MS/MS method provide the concentration of 25(OH)D2 and 25(OH)D3. Neither reference nor interpretative ranges have been established for these analytes.
Question 15. What are vitamin D2 and vitamin D3?
Vitamin D2 is derived from fungal and plant sources and is commonly found in supplements, such as multivitamins, in the United States. Vitamin D2 may also be used in the pharmacological treatment of vitamin D deficiency. Vitamin D3 is derived from animal sources and is made in the skin following exposure to sunlight. The LC/MS/MS technique is able to directly quantify vitamin D2 and vitamin D3. By comparison, immunoassay-based vitamin D tests can only indirectly measure vitamin D2 and vitamin D3; therefore, only the total vitamin D is reported.
Question 16. How can someone obtain vitamin D testing from Quest Diagnostics?
In most states, a physician must provide a patient prescription for testing to be performed. A patient may present a prescription and have a blood specimen collected at any one of our approximately 2,200 Patient Service Centers. Alternatively, blood specimens may be collected at physician offices, clinics, hospitals, or other healthcare facilities.
There are no special patient preparations prior to testing.
Question 17. How can a Patient Service Center appointment be made?
An appointment can be made online at https://secure.QuestDiagnostics.com/hcp/psc/jsp/SearchLocation.do. Alternatively, an appointment can be made by calling 888-277-8772.
Quest Diagnostics is the only national diagnostic testing company that provides nationwide appointment scheduling.
Question 18. What does testing involve?
Once a physician writes a prescription for testing, patients are recommended to visit one of our convenient Patient Service Centers to provide a blood specimen for testing. The patient can make an appointment before visiting the Patient Service Center. Quest Diagnostics is the only national diagnostic testing company that provides nationwide appointment scheduling. To make an appointment, visit https://secure.QuestDiagnostics.com/hcp/psc/jsp/SearchLocation.do
There are no special patient preparations prior to testing.
Question 19. Is fasting required prior to vitamin D testing?
No. There are no special patient preparations prior to testing.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrin Metab. 2011;96:1911-1930. https://www.endocrine.org/~/media/endosociety/Files/Publications/Clinical%20Practice%20Guidelines/FINAL-Standalone-Vitamin-D-Guideline.pdf
Document FAQS.163 Version: 1
Version 1 effective 05/04/2015 to present