Vitamin D Blog: Is a Popular Test Flawed? – MedPage Today
There’s been not one, but two interesting vitamin D studies to come out of the Journal of Clinical Endocrinology & Metabolism: one takes on the reliability of a common test used for vitamin D intoxication, while another looks at free 25-hydroxyvitamin D levels in postmenopausal black and white women.
In the other study, Colin Hawkes, MD, at the Children’s Hospital of Philadelphia, and colleagues took a closer look at a common assay, the DiaSorin radioimmunoassay (RIA), in two case reports, and found that the assay influenced measurement and might not be reliable.
Two adolescents — a 15-year-old male with a 2-week history of postprandial vomiting and a 17-year-old female with a history of weight loss and hypercalcemia — presented to the hospital with vitamin D intoxication and hypercalcemia, according to the authors.
They noticed that both patients had elevated levels of 25(OH)D. But they also had higher levels of serum 1,25-dihydroxyvitamin D (1,25(OH)2D), which didn’t make sense in the context of the vitamin D intoxication they’d been diagnosed with, since intoxication is marked by elevated 25(OH)D levels but suppressed 1,25(OH)2D levels.
“Because of this, the two cases reported here … underwent extensive investigations for alternative or confounding disorders,” the authors wrote.
They took samples from a random set of patients and used two methods to measure levels in vitro: the DiaSorin assay, and liquid chromatography and tandem mass spectrometry (LC-MS/MS). They found that with the assay, median 1,25(OH)2D increased by 114% versus only 21% with LC-MS/MS with the addition of 100 ng/ml 25(OH)D3 and 349% and 117% respectively with 700 ng/ml 25(OH)D3.
After more tests, the researchers concluded that it appeared that 25(OH)D was interfering with the RIA and LC-MS/MS assays for 1,25(OH)2D. “We suspect that the effect is most likely due to assay interference,” they wrote. “While interference was seen with both assays in vitro, it is possible that interference is more relevant clinically in the conventional DiaSorin RIA.”
The authors recommended “measurement of serum 24,25(OH)2D and use of LC-MS/MS, which appears less susceptible to this interference, to reassess serum levels of 1,25(OH)2D when the clinical scenario is confusing.”
Misleading D Levels in Black Women
In the first, researchers led by John Aloia, MD, at Winthrop University Hospital in New York City, looked at 164 healthy, postmenopausal women and found that total vitamin D levels were lower in black than they were in white women. But free, or “bio-available,” vitamin D levels were nearly identical in the two groups (5.25 ± 1.2 versus 5.25 ± 1.3 ng/ml).
Previous studies have established that African Americans have lower vitamin D levels. But they also have superior bone health, which is a paradox, according to the authors. A previous study found that elderly African-American women were only half as likely as elderly white women to experience a fracture, they pointed out.
“Their superior bone health alone should be sufficient to recognize that at the present time African Americans should not be subjected to targeted screening for ‘low’ total serum 25(OH)D,” the authors wrote. They suggested that one reason for the paradox is that free vitamin D levels contribute to bone health. As a result, “the paradox of low total serum 25(OH)D not having an adverse effect on bone health appears to have been partially unraveled.”
The data were taken from previous clinical trials at the hospital in Winthrop. Black and white patients were matched one-to-one based on age and body mass index. Researchers also measured vitamin D binding protein (VDBP) and found that it was significantly lower in blacks when a monoclonal antibody was used (151.4 ± 73.2 mcg/ml versus 264.8 ± 95.5 mcg/ml).
But when they used a polyclonal antibody assay, black participants had higher values of VDBP than did white participants (438 ± 248 mcg/ml versus 369.9 ± 105.3 mcg/ml). They wrote that the absence of any correlation between the two methods was “disturbing” and supported using a direct assay to measure free vitamin D levels, such as ELISA, which they also did.
The findings might mean that African Americans might be incorrectly considered vitamin D deficient with a serum assay that looks only at total vitamin D levels. “In considering the adequacy of vitamin D nutrition, the lower total serum 25(OH)D in African Americans may be misleading,” they added.
They noted that the small sample size was a central weakness of their study.
Other posts from the Vitamin D Blog:
- Vit D Blog: Fortification Coming to New Foods
- No Relief from Menopause With Supplements
- Low Levels For Pregnant Women in Mediterranean
Have a tip on a vitamin D study? Email Parker Brown at email@example.com, or catch our vitamin D feed on Twitter,@vitaminDblog.
The study by Hawkes’s group was supported by the National Institute of Diabetes and Digestive and Kidney Diseases, the National Center for Research Resources and the National Center for Advancing Translational Sciences, NIH.
Hawkes was supported by the National Children’s Research Centre, Dublin, Ireland.
Hawkes and co-authors disclosed no relevant relationships with industry.
The study by Aloia’s group was supported by the NIH.
Aloia and co-authors disclosed no relevant relationships with industry.