Researchers say newer tests tend to overestimate the number of people who are deficient in vitamin D, a problem that could cause patients to be anxious about their health and may lead to over-treatment.
The study, which was presented at ENDO 2012, the annual meeting of the endocrine society in Houston, is adding to concerns felt in many laboratories and hospitals around the country that the results of vitamin D tests, which have become some of the most frequently ordered blood tests in medicine, are widely unreliable.
What's more, experts say, vitamin D testing is often ordered under circumstances where there's little solid scientific evidence to support its use.
"It's a huge problem," says researcher Earle W. Holmes, PhD, a pathologist at Loyola University Stritch School of Medicine, in Chicago.
Experts who were not involved in the research agree.
"The biggest problem is that they're not even consistent," says Ravinder J. Singh, PhD, director of the Mayo Clinic's Endocrinology Laboratory in Rochester, Minn. "At least if they were consistent, you could say, values are half or double, compared to other tests. You can never have confidence" in the results, he tells WebMD.
The new tests, made by Abbott and Siemens, were approved by the FDA last fall.
They're part of a wave of faster, less expensive tests designed to help laboratories keep up with a boom in demand for vitamin D testing.
Abbott took issue with the study results, pointing to a company-funded study that showed their vitamin D test delivered results that are accurate and comparable to a widely used reference method.
Siemens also defended their test, saying they are reviewing the validity of the study.
Questions Persist About Vitamin D Testing
It's not the first time questions have been raised about the accuracy of vitamin D tests, however.
In 2009, Quest Diagnostics, one of the largest medical laboratories in the U.S., sent letters to thousands of doctors warning them that problems with an in-house test may have resulted in inaccurate results being reported to patients. At the time, industry analysts called it the largest patient recall of a laboratory test in recent memory. The company offered free retesting.
Vitamin D, the "sunshine vitamin," plays a well-known and important role in bone health. But in recent years, a raft of research has suggested that low vitamin D may be a factor in a host of other health conditions, including depression, diabetes, cancer, heart disease, and autoimmune diseases.
As a result, doctors have increasingly ordered the test to check vitamin D levels in their patients. The number of vitamin D tests has increased six- to 10-fold over the last decade at some hospitals and laboratories. Industry analysts say the vitamin D tests are now one of the most frequently ordered lab tests in medicine.
"The rate of testing has gone up over the last five to 10 years in an almost exponential fashion," says Naveed Sattar, MD, PhD, a professor of metabolic medicine at the University of Glasgow, in Scotland. Sattar wrote a recent editorial on vitamin D testing for The Lancet, but he was not involved in the research.
Sattar says vitamin D testing has raced ahead of the science, leaving doctors in the dark about how to handle abnormal results.
"Many results are coming back in patients who appear otherwise healthy, but they appear to have low levels. And the physicians don't know what to do, whether they should supplement or not," Sattar says.
Sattar says there's little evidence to suggest that supplementing vitamin D is useful, except perhaps as a treatment for bone loss. The recommended dietary allowances for vitamin D in adults up to 70 years old is 600 International Units (IU) and 800 IU for adults over 70.
While the NIH states a tolerable upper intake level of 4,000 IU, Sattar says that back in the 1950s, when vitamin D was first added to foods, the fortification process wasn't closely monitored and some infants developed dangerously high blood levels of calcium as a result.
Beyond physical harm, however, Sattar says telling someone they are deficient in vitamin D could cause needless anxiety.
"If it were me, I'd be scratching my head. Am I doing something wrong? Should I go spend more time in the sun? Should I be eating certain foods? So that's a hazard by itself," he says.
Someone who looks deficient in D might be prescribed supplements and further testing, increasing costs.
"I think this does need to be looked at carefully," Sattar says.
Holmes and his team wanted to see how well the new tests performed compared to an older, more expensive, and more time-consuming reference method. They were hoping the hospital could switch to one of the newer tests to save money and time.
They ran blood samples from 163 patients on all three tests. The Abbott Architect test was outside an acceptable margin of error -- meaning that the results were either 25% too high or too low, about 40% of the time. The Siemens Centaur2 test was either too high or too low in 48% of samples. In many cases, the newer tests showed that patients were deficient in vitamin D when the reference test indicated they were not.
The new tests use blood proteins called antibodies that bind to vitamin D. They're faster because they look for vitamin D in samples of whole blood.
In the older, reference method, vitamin D is separated from the blood and measured. The older test can also measure two different forms of vitamin D: Vitamin D2, which is the form of the vitamin found in fortified foods and in the kind of high-potency supplements that doctors prescribe to treat patients; and Vitamin D3, the form of the vitamin that the body makes naturally after skin is exposed to sunlight. The newer test can't distinguish between the two different types of D.
Holmes says vitamin D2 seems to confuse the tests.
He says the tests' inability to accurately measure that form of the vitamin means that doctors can't tell if their patients are getting any benefit from it or if they're taking their supplements as directed.
"You can't tell if you're making a difference for the patients," Holmes says.
In absolute numbers, the reference test showed 33 patients out of 163 were deficient in vitamin D, while the Abbott test showed 45 people were vitamin D deficient, and the Siemens test pointed to deficiency in 71 patients.
Current guidelines by the Institute of Medicine state a vitamin D level of at least 20 nanograms per milliliter (ng/ml) in the blood as adequate for bone health and overall health. However, other experts, including Holmes, feel that a normal level is 30 ng/ml or higher.
In a statement issued in response to the study, Abbott, the company that makes the Architect test, says the results of the current study depend on the use of a reference test that is "a very hands-on, labor intensive, manual procedure for which there currently is no standard protocol or calibration method. As a result, this leads to variability of results from lab to lab."
"In current studies ... as well as in recent proficiency surveys, the ARCHITECT Vitamin D test showed excellent precision and reproducibility of results from lab to lab," the statement reads.
Siemens is checking the study's accuracy: "Siemens Healthcare Diagnostics is committed to delivering high-quality solutions and services to our customers that enable health care professionals to accurately diagnose, treat, and monitor patients. We are closely reviewing the validity of the claims made in this report along with the design of the methods and analysis used to develop them."
The wholesale price of the newer tests is estimated to cost around $20 to $25, although patients and insurance companies may be charged more. That's about half the cost of the older reference test, experts said, which wholesaled for about $50 a test.
This study was presented at a medical conference. The findings should be considered preliminary, as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.