Vitamin D is an important fat-soluble vitamin involved in a variety of bodily functions. A blood test to analyze the levels of vitamin D in the blood is an effective way to detect a variety of abnormalities of the bone or digestive system as well as the effectiveness of vitamin supplementation. Understanding the reason for and results of a vitamin D blood test is important in opening the line of communication with a doctor.
Vitamin D Functions
Bones rely on vitamin D for a variety of functions, particularly its role in calcium and phosphate regulation. Vitamin D is required for the body to absorb calcium -- a mineral vital to bone growth and health. It also regulates the levels of phosphate and calcium for correct bone mineralization and is needed by osteoblasts and osteoclasts -- the primary cells involved in bone metabolism. Vitamin D also supports the proper functioning of the immune and nervous system, according to the National Institutes of Health Office of Dietary Supplements.
Test
A vitamin D test is performed for a variety of reasons. Primarily it is used to determine if vitamin D is the cause of bone diseases or calcium metabolism abnormalities as determined by high or low levels of vitamin D. It is also used as an indicator of fat absorption problems after bariatric surgery or in diseases such as cystic fibrosis or Crohn's disease, according to the website Lab Tests Online. The two forms of vitamin D that can be analyzed are 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D. 25-hydroxyvitamin D is the primary form measured as it is a precursor to the other form of vitamin D and is found in higher concentrations in the blood. For the test, a small vial of blood is taken from a vein in the arm through a small needle.
Results
According to MedlinePlus, an online resource of the National Institutes of Health, a normal value for a 25-hydroxyvitamin D test is in the range of 30 to 74 nanograms per milliliter of blood. It is important to note that reference ranges may vary slightly among different laboratories so it is important to discuss all test results with a doctor. Low levels of vitamin D may indicate inadequate sunlight exposure, malnutrition and malabsorption, diseases of the kidney or liver, and negative side effects from medications. High levels of vitamin D are generally caused by overusing prescription vitamin D supplements and can lead to high levels of calcium in the blood, which can damage bones.
Sources of Vitamin D
Vitamin D is available from the diet, through dietary supplements or exposure to sunlight. Ultraviolet light exposure from the sun causes the body to make vitamin D in the skin. The amount of sunlight needed varies depending on geographic location as well as skin color. Many foods are fortified with vitamin D because few foods actually contain sufficient amounts of vitamin D. Non-vitamin D fortified foods include fish, such as tuna, mackerel and salmon, and eggs. Common vitamin D fortified foods include cereals, milk and milk-derived product, such as yogurt and cheese, according to the Office of Dietary Supplements.
Risks
Few risks are associated with drawing blood from the arm and the benefits of analyzing vitamin D levels generally outweigh the risks. A needle is inserted into the arm during a blood draw, which may cause minor pain and irritation at the site of entry. Depending on the individual, it may take several sticks with the needle to correctly locate and draw blood from a vein. There is a risk of infection, excessive bleeding or dizziness and fainting from drawing blood, according to MedlinePlus.
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