Two Vitamin D Immunoassays Compared

Vitamin D can be synthesized in skin from 7-dehydrocholesterol following ultraviolet light exposure, which produces Vitamin D3 (cholecalciferol). Vitamin D2 (ergocalciferol) is present in plant sources, and can be absorbed from the diet.

Measurement of 25-Hydroxyvitamin D (25-OH-D) is commonly performed to assess Vitamin D status in clinical laboratories; however, the ability of these assays to detect Vitamin D2 (as 25-OH-D2) or Vitamin D3 (as 25-OH-D3) varies. Assessment of Vitamin D status by measurement of 25-Hydroxyvitamin D (25-OH-D) is widely performed by immunoassay.


Biochemists at Baylor College of Medicine (Houston, TX, USA) and their colleagues evaluated the performance of the institution's total 25-OH-D assay on the Architect i1000 SR platform to detect either 25-OH-D2 or 25-OH-D3. This assay is a chemiluminescent microparticle immunoassay. Samples spiked with either 25-OH-D2 or 25-OH-D3 and reference laboratory samples with either 25-OH-D2 or 25-OH-D3 concentrations determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS) were used for evaluation. As a comparison to the Architect assay, samples were also tested on the Cobas 8000, which uses an electrochemiluminescence binding assay.

The scientists reported that recovery of 25-OH-D3 in spiked samples was similar by Architect (84%–87%) and Cobas (90%). Recovery of 25-OH-D2 was lower than 25-OH-D3, and was poorer by Architect (37%–40%) than by Cobas (69%–71%). In measurement of samples with known 25-OH-D concentrations, performance of Architect and Cobas assays was similar for 25-OH-D3. However, at concentrations greater than 50 nmol/L 25-OH-D2, the Architect assay exhibited large average negative bias of −27%.

The authors concluded that while the Architect and Cobas assays performed similarly in detection of 25-OH-D3, the Architect assay was significantly poorer at detecting 25-OH-D2 than Cobas, with poorer recovery and significant negative bias at higher concentrations of 25-OH-D2. This agrees with other studies, and indicates that caution should be used in interpreting Architect 25-OH-D results in patients supplemented with Vitamin D2. The study was published on July 29, 2019, in the journal Practical Laboratory Medicine.