Newer Vitamin D Tests Often Inaccurate: Study
Faster, Less Expensive Tests May Overestimate Vitamin D Deficiency
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Questions Persist About Vitamin D Testing continued...
"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.