Newer Vitamin D Tests Often Inaccurate: Study
Faster, Less Expensive Tests May Overestimate Vitamin D Deficiency
WebMD News Archive
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.