Efforts to reach agreement on how vitamin D deficiency is defined are complicated by the fact that the cutoff points used in reports from clinical laboratories vary widely.
“I think reporting is a great problem because primary care physicians are very hurried,” Dr. John F. Aloia said at a public conference on vitamin D sponsored by the National Institutes of Health. “When you look at the laboratory report, what you get is a column that’s normal and another column that’s low or high. The choice of the laboratories to choose their own cutpoints is really a problem. The other part of that reporting is using the low level of normal in a range at the RDA [recommended daily allowance].”
In its recently updated recommendations on vitamin D screening, the U.S. Preventive Services Task Force noted that variability between serum vitamin D assay methods “and between laboratories using the same methods may range from 10% to 20%, and classification of samples as ‘deficient’ or ‘nondeficient’ may vary by 4% to 32%, depending on which assay is used. Another factor that may complicate interpretation is that 25-(OH)D may act as a negative acute-phase reactant and its levels may decrease in response to inflammation. Lastly, whether common laboratory reference ranges are appropriate for all ethnic groups is unclear.”
Trying to exert influence on what ranges of serum vitamin D laboratories are using in reporting data “is an issue,” said Dr. Aloia, director of the Bone Mineral Research Center at Winthrop University Hospital, Mineola, N.Y., and professor of medicine at Stony Brook (N.Y.) University. “A laboratory can report anything it chooses to. For instance, the American College of Pathology and other [professional organizations] don’t have the responsibility for [the cut-offs in] those reports.”
Dr. Aloia favors translating the reporting of vitamin D levels based on something like Z scores, “so when you see lab reports, some of them will have a paragraph of explanation to guide the physician,” he explained. “We’re going to need that. We have to move away from just [a] cutpoint range and the lower level of the range being the RDA.”
Dr. Roger Bouillon, professor emeritus of internal medicine at the University of Leuven (Belgium), supports a threshold of 20 ng/mL serum vitamin D in adults. “I don’t like a range [of vitamin D]; they just need to have a level above 20 ng/mL. For me, a threshold is the best strategy on a population basis.”
During an open comment session, attendee Dr. Neil C. Binkley expressed concern over applying Z-score principles to the vitamin D field. “I love bone density measurement,” said Dr. Binkley, codirector of the Osteoporosis Clinical Center & Research Program at the University of Wisconsin, Madison, and past president of the International Society for Clinical Densitometry. “The T-score was in fact an advance in the field. But I can’t tell you how strongly I would urge you to not consider T-scores or Z-scores or something like that in the vitamin D field. Rather, I would urge that we do a better job at measuring 25-hydroxyvitamin D so our laboratories agree and have concise guidance for primary care. If you choose to go into the probability realm and the Z-scores, it is going to be a disaster.”
The presenters reported having no financial disclosures.