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Automated vitamin D assays versus traditional methods


Laboratories around the world are moving over to automated methods of assaying serum levels of 25-hydroxyvitamin D (vitamin D); Barake et al. used a traditional Immunodiagnostics Systems RIA analysis (IDS-RIA) and the DiaSorin Liaison assay (Liaison) automated system in the same patient group to assess how the two compared. Serum vitamin D levels were measured in 494 patients, and a subset of 83 samples was used to investigate inter-assay variability.

The automated Liaison system consistently reported lower serum vitamin D levels with a mean bias of −5 ng/ml. This led to significantly different recommendations for treatment. When 20 ng/ml was used as the cutoff, 52% compared to 36% of patients (Liaison vs. IDS-RIA) would have been treated. At the higher cutoff point of 30 ng/ml, 79% of patients were assessed as treatable by Liaison, compared to 64% using the traditional assay.

Editor's comment: This study confirms that differences in results between serum 25-hydroxyvitamin D assays exist and are large enough to affect clinical decision making. If this is true in a specialty laboratory, imagine what might happen in a routine clinical laboratory available to most physicians. The debate about whether an appropriate target level of serum 25-hydroxyvitamin D for optimal musculoskeletal health is 50 or 75 nmol/l becomes irrelevant if we cannot analyse serum levels accurately, reproducibly or confidently.

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