What it measures
Active B12, also called holotranscobalamin or holoTC, is the part of vitamin B12 in the blood bound to transcobalamin, the carrier that delivers it to cells. A total B12 test also counts B12 on a carrier cells cannot use, so some labs use active B12 when a total B12 result is hard to read.
Why people test it
- Checked when tiredness, pins and needles, a sore tongue or large red blood cells suggest low B12.
- Measured in people on vegan or vegetarian diets, as B12 comes mainly from animal foods.
- Checked in people on medicines linked to low B12, such as metformin or long-term acid-reducing tablets.
- Used in pregnancy, when total B12 can fall and be harder to read.
- Checked by some endurance athletes alongside ferritin when looking into tiredness.
Typical range
Ranges vary between labs with the method used, age and sex, and cut-offs for active B12 differ more than most, so your report's own range is the one that applies. One NHS trust's lab reads below 25 pmol/L as consistent with deficiency, 25 to 70 as indeterminate and above 70 as deficiency unlikely. Source: North Bristol NHS Trust, Severn Pathology: Active B12/HoloTC.
Finger-prick or vein?
A finger-prick sample is usually fine for active B12.
B12 supplements and injections raise active B12, so a result taken while using them reflects that intake.
