June 28, 2012
Newly Approved Tests on Vitamin D Are At Least 40 Percent Inaccurate
Inaccuracies in vitamin D blood tests may be confounding efforts to establish optimal recommended levels for health, warn researchers.
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.
Testing for vitamin D levels, once uncommon, has skyrocketed as medical studies raise awareness about vitamin D deficiencies, according to three of the USA's largest medical diagnostic labs. Physicians agree that they're increasingly using the blood test to find out whether their patients are low on the vital vitamin.
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.
About vitamin D
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.
There are a number of different companies that have approval to perform vitamin D testing, but the gold standard is DiaSorin. Their radioimmunoassay (RIA) method for measuring total vitamin D levels has become the gold standard, not because it’s more accurate than the others, but because it’s the one used in almost every major vitamin D study, on which the recommended blood levels for clinical efficacy are based.
Therefore, in order for any other testing method to offer clinically relevant results, the test values must agree with DiaSorin RIA results, since those were used to establish the recommended levels.
Vitamin D status is measured by looking at blood levels of 25-hydroxyvitamin D3. There are three common methods used for measuring vitamin D3:
1. LC-MS/MS -- This test measures 25-hydroxyvitamin D2 and D3 separately
2. RIA (DiaSorin) -- Developed in 1985, it accurately measures total 25-hydroxyvitamin D (It does not separate D2 and D3)
3. Liaison (DiaSorin) -- a more recently developed automated immunoassay by DiaSorin that has largely replaced the RIA
The LC-MS/MS (liquid chromotatography-mass spectrometry) method is the preferred method for many labs, including the Mayo Clinic, Quest Labs, Esoterix, ZRT, and others, while Liaison is favored by other testing labs like LabCorp.
Since the DiaSorin assay (RIA) was used in the major clinical studies that led to the recommended vitamin D levels, any lab using the LC-MS/MS method need to make sure their test correlate with the RIA test values in order to accurately determine your vitamin D status.
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."
Marco Torres is a research specialist, writer and consumer advocate for healthy lifestyles. He holds degrees in Public Health and Environmental Science and is a professional speaker on topics such as disease prevention, environmental toxins and health policy.