Vitamin D

Vitamin D is now recognized not only for its importance in preventing rickets, but also in preventing osteopenia, osteoporosis, muscle weakness, and falls. Testing levels ofvitamin D can be considered in patients at increased risk of vitamin D deficiency, including elderly patients and those with osteoporosis, osteopenia, fat malabsorption, chronic kidney disease, and increased skin pigmentation. The term "vitamin D" includes vitamin D2 and vitamin D3. Vitamin D2 (calciferol) is manufactured from the plant sterols in yeast, and vitamin D3 (cholecalciferol) is manufactured from lanolin. Vitamin D is hydroxylated by the liver into 25-hydroxyvita-min D [25(OH)D], the major circulating form of vitamin D in the body. The kidney converts 25(OH)D into 1,25-dihydroxyvitamin D [1,25(OH)2D], which is the active form of vitamin D.

The complete reference list is available online at www.expertconsult.com.

www.nlm.nih.gov/medlineplus/laboratorytests.html

An overview for patients that explains basic laboratory principles, also gives links to other resources. www.labtestsonline.org/understanding/analytes/microalbumin/test.html Developed by American Association for Clinical Chemistry, site allows patients to search for explanation of specific tests.

The laboratory diagnosis of vitamin D deficiency relies on measuring the levels of 25(OH)D. Measuring 1,25(OH)2D is not recommended in clinical practice because it is not a reliable indicator of vitamin D status. 1,25(OH)2D has a half-life of only 4 hours, whereas 25(OH)D has a halflife of about 3 weeks. In addition, vitamin 1,25(OH)2D levels can actually increase with vitamin D deficiency, because increasing PTH levels result in increasing levels of 1,25(OH)2D.

The 25(OH)D is a measurement of vitamin D intake and that made in the body after sun exposure. Although some laboratories may report 25(OH)D2 and 25(OH)D3 levels, it is the total level that is used clinically to monitor vitamin D status. PTH rises when the 25(OH)D levels are less than 30 ng/mL. Although labs may list 20 to 100 ng/mL as the reference range for 25(OH)D, most experts define a preferred level of 25(OH)D as 30 to 60 ng/mL, with deficiency defined as less than 20 ng/mL and insufficiency as 20 to 20 ng/mL. Currently, a 25(OH)D greater than 30 ng/mL is considered the preferred level for both adults and children.

www.ahrq.gov/CLINIC/uspstfix.htm

Homepage of the U.S. Preventive Services Task Force (USPSTF), which lists recommendations for screening tests.

References

Web Resources

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