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24. HoloTC (Holotranscobalamin) measures which of the following ??
1. Active B12
2. Inactive B12
3. Total B12
4. None of these
Answer = 
When B12 deficiency is suspected, several circulating biomarkers can be used to confirm or dismiss the diagnosis.
These markers include two direct markers (i.e., total B12 and active B12, which is referred to as holotranscobalamin (HoloTC)) and two metabolic markers (i.e., homocysteine (Hcy) and methylmalonic acid (MMA).
The determination of total B12 relies on the integral measurement of two B12 fractions:
 Biologically active HoloTC (i.e., B12 bound to transcobalamin)
 Biologically inactive holohaptocorrin (i.e., B12 bound to haptocorrin)