These two arrive on reports under names close enough to be mistaken for each other, and they measure different quantities. If you are tracking B12 across labs, this is the distinction most likely to corrupt the history.
Total vitamin B12
MedlinePlus describes the standard test simply: the vitamin B12 level "is a blood test that measures how much vitamin B12 is in your blood."
That is all of it in circulation — including the portion bound to carrier proteins that do not deliver it to your cells.
Active B12
Also printed as holotranscobalamin or HoloTC. It measures only the fraction bound to the carrier that makes B12 available to tissue — the portion your cells can actually take up.
Because it is a subset of the total, its numbers are smaller and its reference interval is different. A total B12 result and an Active B12 result are not comparable figures, and neither is a check on the other.
Why a second test exists
Because the total includes B12 that is present but not usable. A total measurement can therefore be less informative than it looks, which is why other tests are used to resolve an unclear picture.
MedlinePlus notes that a low result is not treated as the end of the question: it describes confirming deficiency "by checking the level of a substance in the blood called methylmalonic acid". So a further, separate test exists and is sometimes ordered alongside.
What any of those results mean together is a clinical matter and belongs with a doctor. The point here is only structural: a single B12 row is frequently not the whole investigation, and a report showing one is not showing you the rest of it.
The practical problem for a history
Reports do not always make the distinction obvious. A row reading "Vitamin B12" at one laboratory and "Active B12" at another are different tests, and plotted on one line they produce a dramatic drop that is purely a change of test.
The units differ too, which is the most reliable tell available on the page — along with the reference interval, which will be conspicuously different between the two.
How Arkalife treats them
As two separate markers, deliberately, with their own histories and their own charts. Merging them would produce a single continuous-looking trend out of two incompatible measurements — which is exactly the failure this guide exists to describe.
If a printed name is genuinely ambiguous about which was run, the reading is stored and displayed as the lab printed it and left unmapped rather than filed against a marker it may not belong to.