WHO/BS/2015.2263 Report of the international collaborative study to assign a holotranscobalamin (holoTC) value to the IS for folate and B12 (03/178)

Overview

There is evidence that holotranscobalamin (holoTC), the active portion of B12, is a better marker of early B12 deficiency than total B12. The recent availability of commercial assays for holoTC prompted an international collaborative study to assign a holoTC value to the 1st International Standard (IS) for serum folate and B12, 03/178.

The IS, 03/178, was assayed by 12 laboratories in 8 countries using manual and automated immunoassays for holoTC; one laboratory additionally performed an in-house assay. Fourteen sets of data were analysed.

An overall geometric mean value of 106.8 pmol/L was obtained, with an inter-laboratory GCV of 10.5%. The inclusion of three serum samples in the study, with different holoTC levels, demonstrated a reduction in inter-laboratory variability when the holoTC levels in the samples were determined relative to the IS (holoTC value of 106.8 pmol/L assigned) rather than to the assays’ calibration.

It is recommended that a holoTC value of 107 pmol/L is assigned to 03/178 for use as the 1st International Standard for folate, B12 and holoTC.