Either test is a suitable first check for B12, and active B12 is the one to choose in pregnancy and a sensible choice on a vegetarian or vegan diet. At Blood London, the active B12 test is £96.25 with results in 2 days, and total B12 is £75 with results in 1 day, and you can walk in at 117a Harley Street any day of the week. Each carries the £50 blood draw once per visit, so the totals are £146.25 and £125.
Total B12 counts all the vitamin B12 in your blood. Active B12 counts only the part your cells can take up and use. The reason there are two tests is a carrier protein. Most B12 in the blood is bound to a carrier that cells cannot use, and a smaller share travels on a different carrier, holotranscobalamin, which delivers it to the cells. Total B12 adds both together. Active B12 measures the delivered share.
NICE guideline NG239 (2024) says either test is a suitable first check when B12 deficiency is suspected. The sections below match common situations to a test.
A first check for tiredness or tingling
Total B12 is cheaper and a day quicker, which suits a straightforward first look. Active B12 costs £21.25 more and answers a narrower question: how much B12 is available to your cells. The exception is recreational nitrous oxide use, where both B12 results can look normal; NICE suggests homocysteine or methylmalonic acid (MMA) instead.
If tiredness is the main worry and you have no idea why, our sister clinic's guide to why you might be tired all the time lists the other common causes.
A vegetarian or vegan diet
Plant foods contain very little natural B12, so people who eat no meat, fish or dairy often check their level. Active B12 is a sensible choice here because it shows what your cells can reach. Folate works alongside B12, and active B12 with red cell folate is £145.63 from one sample, or £195.63 with the draw. Results take 2 days.
Pregnancy or breastfeeding
NICE recommends active B12 as the initial test during pregnancy. Total B12 can fall in pregnancy without the cells running short, so the active result is the steadier guide. Mention a pregnancy when you arrive and choose active B12.
When the result is borderline
Plenty of results land in a middle band. NICE calls a total B12 between 180 and 350 ng/L (133 to 258 pmol/L), or an active B12 between 25 and 70 pmol/L, indeterminate. For a borderline result NICE says to consider a further test, serum methylmalonic acid (MMA), when there are symptoms or signs.
"Plenty of people bring me a total B12 that's just under the line. I repeat it as active B12, and if that's borderline too, we look at the full picture before deciding anything."
Dr Mohammad Bakhtiar, Clinical Lead, GMC 4694470
At Blood London, methylmalonic acid (serum) is £303.75 with results in 5 days. If your doctor suspects the stomach is involved, gastric parietal autoantibodies is £70 with results in 2 days. Add the single £50 draw for the visit, and an optional £75 doctor review if you would like the results read together. Bring any earlier result so the numbers can be compared. During pregnancy or breastfeeding, NICE says a doctor can consider starting treatment while the MMA result is awaited.
Known anaemia, or a wider picture
When blood counts or tiredness point to anaemia, B12 is one of several things to check. The Anaemia Profile is £226.25 with results in 2 days. It includes active B12, red cell folate, ferritin, iron with TIBC, a full blood count and ESR.
Bought separately, the same six cost £96.25 + £68.13 + £68.75 + £43.75 + £63.75 + £43.75 = £384.38. With one £50 draw, that is £434.38 against £276.25 for the profile, a difference of £158.13. Our guides to ferritin and iron studies and full blood count results explain two of those markers.
A repeat check after B12 treatment
Choose the same test you had before, so the two results compare directly. Ask the doctor who started your treatment when they want it repeated.
When a doctor adds intrinsic factor antibodies
Intrinsic factor is a protein made in the stomach that helps the body absorb B12. The intrinsic factor antibodies test looks for antibodies against it. It is £141.25 with results in 3 days, and a doctor may suggest it when a low B12 has no clear dietary reason.
What the visit involves
At Blood London, the draw takes place at 117a Harley Street, open Monday to Friday 09:00 to 18:00 and Saturday and Sunday 10:00 to 14:00. One £50 draw covers every test taken at the same visit, so adding a second B12 test or folate costs only the test itself. If you would like a GMC-registered doctor to explain your results, an optional review is £75.
Frequently asked questions
What is holotranscobalamin?
It is the medical name for active B12: vitamin B12 attached to the carrier that delivers it to cells.
Does the Anaemia Profile include total B12?
It includes active B12, not total B12.
Should I tell the clinic about supplements or the pill?
Yes. NICE notes that B12 tablets, injections or patches can raise a result without fully treating a deficiency, and that the combined contraceptive pill can lower total B12. Mention both when you arrive, and give blood before any treatment starts.
Book your B12 test
Tell us which situation fits and we will help you choose when you arrive. Walk in any day, or book a private blood test in London online.