The U.S. Preventive Services Task Force (USPSTF), in a draft statement, said it found insufficient evidence to recommend routine vitamin D screening. The independent panel, funded by the U.S. Agency for Healthcare Research and Quality, based this preliminary finding on a number of factors, including a dearth of studies that point to the direct benefits of testing adults for vitamin D deficiencies, and no uniform standards for the testing.
A number of testing methods exist to determine total serum 25-hydroxy vitamin D (25 (OH)D) levels. Yet, “the accuracy of these tests is difficult to determine due to the lack of studies using an internationally recognized reference standard,” according to the draft statement.
It is also difficult to compare results of studies that use different assays “and have different cut-off points as to what is a normal range of values,” observes USPSTF member Linda Ciofu Baumann, PhD, RN, a professor emerita at the University of Wisconsin-Madison School of Nursing and affiliate faculty at the University of Wisconsin School of Medicine and Public Health.
According to the draft statement, “testing variability between methods and between laboratories using the same method has been observed, and whether a patient's sample is classified as ‘deficient’ or ‘nondeficient’ has been found to vary by 4% to 32% depending on which assay is used.”
Accuracy of these tests isn't the only problem; it’s also unclear if 25(OH) D is the best way to measure vitamin D status. “The current tests measure the blood serum level of 25-hydroxy vitamin D, not its availability in the body,” Baumann says. Bioavailable 25 (OH) D, by comparison, “is a more accurate reflection of physiological availability of 25-hydroxy vitamin D,” she says.
The task force additionally noted the difficulties in evaluating effects of vitamin D levels on health outcomes. The risk for developing various health problems, such as falls, fractures, diabetes, cardiovascular disease, depression and even mortality, has often been linked to lower vitamin D levels.
“However, observations of these associations have been inconsistent and may vary depending on the cutpoint used to define low vitamin D levels and by subpopulations (defined by race and institutionalized populations),” according to the draft statement. As an example, African Americans tend to have lower vitamin D levels than whites—but substantially lower rates of reported fractures.
There’s also insufficient evidence to support the benefits of treating asymptomatic vitamin D deficiency on conditions such as physical or psychosocial functioning, according to the draft statement.
According to Baumann, there are instances where it is still appropriate to check for vitamin D deficiency in certain higher-risk individuals. This includes individuals with low vitamin D intake, decreased vitamin D absorption, or little or no ultraviolet B exposure due to the winter season, high latitude, or physical sun avoidance. “Obesity and darker skin pigmentation may also be associated with low levels of total serum 25-hydroxy vitamin D, but it is unclear if this reflects vitamin D deficiency,” she explains.
The public comment period on the draft statement closed July 21. The next step is for the task force to review public comments and disseminate a final recommendation statement, which will be published in the Annals of Internal Medicine.