In serum, vitamin B12 exists in 2 bound forms. Cells can only take up vitamin B12 in the form of holoTC. Therefore, measuring holoTC is more reflective of vitamin B12 status than measuring total vitamin B12 alone (which is widely done in the NHS).
Emerging evidence suggests that a low level of holoTC may be a more reliable marker of vitamin B12 deficiency than a low serum cobalamin level, particularly as an early marker.
We have now added active B12 to our comprehensive functional profile free of charge.
Hunt et al. 2014