June 25, 2012 -- Two new lab tests to measure blood levels of vitamin D are inaccurate more than 40% of the time, according to a new study.
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.
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.