Active Vitamin B12 (Holo-TC)

Vitamin B12 is carried in the blood by two proteins. Most of it is bound to haptocorrin and cannot be used by the cells; a smaller part is bound to transcobalamin. That part, holotranscobalamin or active B12, is the B12 that the cells actually take up. This test measures only that active part and therefore says more about usable B12 than the total B12 value.

• Vitamin B12 deficiency, also at an early stage without anemia
• A deficiency due to too little intake (vegan or vegetarian diet without a supplement) or poor absorption (atrophic gastritis, pernicious anemia, stomach or bowel surgery, Crohn's disease, long-term use of stomach acid inhibitors or metformin)

• Do you take biotin (vitamin B8, often in hair and nail supplements)? Stop at least 2 days before the sample is taken, or 4 days for a high dose.

With a low value, the GP usually checks the blood count and looks for the cause; with a value in the gray zone, methylmalonic acid can give more clarity. If a deficiency is confirmed, the GP usually starts supplementation with tablets or injections.

According to the Dutch Society for Clinical Chemistry (NVKC), active B12 is more sensitive than total B12 for ruling out a deficiency, so methylmalonic acid or homocysteine is needed less often. The usual first measurement when a deficiency is suspected is total B12, with methylmalonic acid as a follow-up test if in doubt; active B12 is an alternative to that. With a value in the gray zone and matching symptoms, one measurement is not enough to confirm or rule out a deficiency.

• B12 supplements, injections and fortified foods raise the value quickly; during supplementation, the result says little about an earlier deficiency.
• Reduced kidney function can raise the value.
• The test is not available in every laboratory, and cutoff values differ by laboratory and measuring method.
• High doses of biotin (hair and nail supplements) can interfere with some measuring methods.

Anemia due to a deficiency of vitamin B12 or folic acid newly occurs in general practice in about 2 in 1000 people per year. A B12 deficiency without anemia is more common, especially at older ages, with a vegan diet without a supplement and with long-term use of stomach acid inhibitors or metformin; exactly how common depends on the test and cutoff value used.

Useful with symptoms that fit a B12 deficiency (tiredness, tingling or numbness in the hands or feet, balance or memory complaints, a sore tongue), with a vegan or vegetarian diet without a supplement, after stomach or bowel surgery, with Crohn's disease, with long-term use of stomach acid inhibitors or metformin, and with nitrous oxide use. If you already take B12 tablets or injections, the value says little about an earlier deficiency. In adults without symptoms who eat a varied diet, the chance of a deficiency is small, and a value in the gray zone mainly leads to extra tests.

• Recent supplementation can cause a deficiency to be missed, because active B12 rises quickly.
• A value in the gray zone can lead to unnecessary worry if no follow-up test or assessment of the symptoms follows.

• NVKC, allesovertesten.nl: Vitamine B12
• NHG-Standaard Anemie (M76)
• H&W 2014: NHG-Standaard Anemie, eerste herziening (openbare volledige tekst)
• NVKC, allesovertesten.nl: Methylmalonzuur
• Thuisarts.nl: Ik heb misschien te weinig vitamine B12