A test for vitamin D that was once reserved for patients with unusual bone fracturing or those at risk for osteoporosis has now found popularity in doctor's offices alongside tests for cholesterol and thyroid hormone during routine bloodwork.
Research has shown that vitamin D is an important nutrient for maintaining strong bones, aids in the movement of muscles and helps the immune system fight off disease. Although vitamin D is produced naturally when skin is exposed to sunlight, lives spent indoors mean that many Americans are vitamin D deficient.
But as the popularity of testing grows, some scientists and regulators caution that vitamin D is difficult to measure accurately.
In addition, no official standardization exists for vitamin D testing -- a single blood sample sent to 10 different labs could return 10 different test results, said Dr. Gary Horowitz, director of clinical chemistry at Beth Israel Deaconess Medical Center in Boston.
The federal government is moving to make the tests more reliable for the doctors and patients who choose to use them, but in the meantime experts say not to put too much stock in the test alone when determining treatment.
"If your doctor thinks it's important to have the test, I would have it," said Christopher Sempos, coordinator for the federal Vitamin D Standardization Program. "But a lot of deciding how to treat an individual is more than just measuring the blood test. You have to trust the physician to use clinical judgment and not just rely solely on a clinical lab test."
Two methods dominate vitamin D testing: commonly used immunoassays that use antibodies to detect vitamin D and an elaborate chromatographic method that separates vitamin D based on chemical properties.
"Immunoassays … allow laboratories relatively inexpensively and relatively simply to make these measurements," said Earle Holmes, professor of pathology and pharmacology at Loyola Medicine in Maywood, Ill. "On the other hand, the (liquid chromatography) method is a much more complicated method that requires more expertise and is a lot more expensive to operate."
Both methods have strengths and weaknesses, but research has shown there can be discrepancies in the results.
"This is a time of great interest in vitamin D, and lots of new methods are going to come into the field," said Holmes, who is studying the performance of vitamin D tests in an ongoing effort to find the best one for his laboratory. "It is our responsibility to test everything to see if it meets our needs."
But neither of these efforts aims to find out how accurate a test is at measuring vitamin D. "If you want to see how close people are to the truth, get the true value and then compare routine methods to that," said Horowitz.
The Vitamin D Standardization Program, launched in 2010 as a joint initiative of the National Institutes of Health, the Centers for Disease Control and Prevention and the National Institute for Standards and Technology, aims to do just that.
"The goal of the program is to standardize any method that's used to measure vitamin D, no matter how it's designed," said Sempos, program coordinator for the NIH Office of Dietary Supplements.