Measurement uncertainty of 25-OH vitamin D determination with different commercially available kits: impact on the clinical cut offs

Due to measurement uncertainty, the true 25-OH vitamin D (25(OH)D) of a patient (whatever the commercially available assay tested) will be > 80 nmol/L if its measured concentration is > 100 nmol/L. Thus, if a physician considers that a normal VTD status is a 25(OH)D level a parts per thousand yen80 nmol/L, he should ensure that the patient's results are a parts per thousand yen100 nmol/L. Many experts recommend that serum levels of 25(OH)D should be above a lower normal limit of 75-80 nmol/L. However, the value delivered by laboratories is only an estimation of the true value due to measurement uncertainty. When using a cut off, measurement uncertainty around the cut off is important because therapeutic actions may differ if the measured value is below or above the limit. We aimed to establish the measurement uncertainty at different levels of concentration for several commercially available 25(OH)D analytical techniques. We constituted three pools of serum with different 25(OH)D concentrations. Each pool was assayed in triplicate during 5 days with the DiaSorin RIA, Liaison, Elecsys, and Chromsystems-HPLC assays. We report a relatively high measurement uncertainty for the measurement of 25(OH)D for the four different techniques: the mean relative uncertainties, all techniques confounded were 19.4%, 16.0%, and 11.3% for pool 1 (35.3 nmol/L), pool 2 (79.5 nmol/L), and pool 3 (126.1 nmol/L), respectively. Our results show that, whatever the assay, the true 25(OH)D of a patient will be > 80 nmol/L if its measured concentration is > 100 nmol/L. In other words, if a physician considers that a normal VTD status is defined by a 25(OH)D level a parts per thousand yen80 nmol/L, he should ensure that the patients present a 25(OH)D a parts per thousand yen100 nmol/L.