The research findings – presented at The Endocrine Society's 94th Annual Meeting – revealed that two newly approved blood tests for vitamin D were inaccurate in at least 40% of laboratory specimens analysed.
Dr Earle Holmes of Loyola University Stritch School of Medicine, USA, and his colleagues examined how well the new tests – Abbott Architect and Siemans Centaur2 – performed on 163 randomly selected blood samples. The team reported that in 40% of the Abbott Architect specimens and 48% of the Siemans Centaur2 specimens, results were inaccurate by over the maximum allowable error of plus-or-minus 25%.
Such inaccuracies could lead to misdiagnoses of patients, said Holmes.
"These inaccuracies also could confound efforts to identify the optimal levels of vitamin D for good health," he added.
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol.
Both D3 and D2 precursors are transformed in the liver and kidneys into 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
Vitamin D deficiency in adults is reported to precipitate or exacerbate osteopenia, osteoporosis, muscle weakness, fractures, common cancers, autoimmune diseases, infectious diseases and cardiovascular diseases.
There is also some evidence that the vitamin may reduce the incidence of several types of cancer and type-1 diabetes.
Testing vitamin D
Holmes noted that doctors and nutritional advisors are increasingly ordering blood tests to measure vitamin D - often referred to as the sunshine vitamin - revealing that such tests are now among the most frequently ordered medical tests.
Such an increase in testing may result from recent scientific reports that a growing percentage of the global population has low vitamin D status, in combination with fresh research to link insufficient vitamin D with higher risks of certain diseases such as diabetes, heart disease, multiple sclerosis, osteoporosis and certain cancers.
The researchers said the new tests tended to overestimate vitamin D deficiency.
According to gold-standard LCMS measurements, 33 of the 163 specimens showed vitamin D deficiency. However the Abbott test showed that 45 specimens had vitamin D deficiency while the Siemens test showed that 71 subjects had vitamin D deficiency.
Such inaccuracies could lead to over diagnosis of vitamin D deficiency, Holmes said.
"There has been an exponential increase in the number of vitamin D tests ordered for patients," said the US-based researcher. "But our study of two newly approved tests showed they had pretty poor performance."