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HOUSTON -- Two widely used tests to determine vitamin D status produced results that exceeded the allowable limits for error in at least 40% of cases, according to a study reported here.

Using liquid chromatography/mass spectrometry (LC/MS) as the standard, variance by the Abbott Architect and Siemens Centaur-2 tests ranged from -60% to +80%. The maximum allowable error for test results is ±25%, Earle Holmes, PhD, said at ENDO 2012.

Errors with both tests tended to be on the side of overestimating the frequency of vitamin D deficiency, he added.

"Inaccurate test results can lead to overdiagnosis and overtreatment, which is both unnecessary and costly," Holmes, of Loyola University in Maywood, Ill., told 番茄社区app. "Vitamin D is one of the most commonly requested laboratory tests, and the use is increasing and more conditions are being associated with vitamin D insufficiency."

Recent studies have demonstrated low levels of vitamin D in association with numerous medical conditions, and the number of links continues to grow. Studies reported at ENDO 2012 showed associations between vitamin D insufficiency and type 2 diabetes, depression, and poor response to osteoporosis drugs.

Such studies have fueled an upswing in clinician orders for vitamin D testing, which in turn has led to development of several new immunoassays for measuring 25 hydroxy vitamin D [25(OH)D] in unextracted serum. Holmes and colleagues examined the accuracy of two newer tests already in wide use.

The study involved 163 randomly selected clinical samples submitted for testing from March 2 to March 7, 2011. The samples were obtained from 123 women who had a median age of 54 and 40 men, median age, 59. Holmes said 98 specimens contained only 25(OH)D3, whereas 65 contained 25(OH)D2 and 25(OH)D3.

For each clinical sample, 25(OH)D was measured by LC/MS performed at an outside laboratory. Holmes and colleagues also evaluated each sample with the Architect and Centaur-2 tests, following manufacturer instructions.

Cutoff values were <20 ng/mL for vitamin D deficiency, 20 to 30 ng/mL for low vitamin D, and ≥30 ng/mL for normal. The test results put the following number of patients in each category of serum vitamin D:

• LC/MS -- 33 deficient, 44 low, 83 normal
• Centaur-2 -- 71 deficient, 32 low, 60 normal
• Architect -- 45 deficient, 62 low, 56 normal

Applying the ±25% allowable margin of error, the investigators found an error rate of 40% with the Architect and 48% with the Centaur-2.

"Both of these new FDA-approved immunoassays were characterized by high degrees of random variability and significant constant biases relative to an established LC/MS method, inaccurate measurement of 25(OH)D, and a failure to meet even a minimum quality standard for analytical bias for many of the clinical specimens we tested," Holmes and colleagues concluded in their poster presentation.

"The inaccuracy of these immunoassays at 25(OH)D levels in the lower part of the analytical measuring range would have led to a marked overestimation of the prevalence of vitamin D deficiency in our study sample.

"The widespread use of new analytical methods with poor analytical quality may confound efforts to establish reference values for 25(OH)D in health and efforts to evaluate the role of vitamin D insufficiency as a risk factor in disease."

In response to a request from 番茄社区app, a Siemens spokesperson said via email, "Siemens Healthcare Diagnostics is committed to delivering high-quality solutions and services to our customers that enable healthcare professionals to accurately diagnose, treat, and monitor patients.

"We are closely reviewing the validity of the claims made in this report, along with the design of the methods and analysis used to develop them."

In a prepared statement, Abbott representatives said, "In current studies ... as well as in recent proficiency surveys, the Architect Vitamin D test showed excellent precision and reproducibility of results from lab to lab. This is very important given the increase in volume of vitamin D testing due to the critical role of vitamin D in several disease states.

"The Holmes study used the LC/MS method, which is a very hands-on, labor-intensive, manual procedure for which there currently is no standard protocol or calibration method. As a result, this leads to variability of results from lab to lab. While LC/MS may be the appropriate method for Dr. Holmes' lab, based on his patient population, it may not be the test of choice for a lab with a typical patient population, as seen worldwide."

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