Measuring vitamin B12 levels

MEASURING VITAMIN B12 LEVELS IN LABORATORY

Vitamin B12 levels are measured from the blood serum. Assessing the total levels of vitamin B12 is complicated due to it binding to both active transcobalamin (S-B12-TC2), 10–20 %) and inactive haptocorrin (S-B12-HC, 80–90 %) in the blood. The combined levels of these are measured. The specification of active vitamin B12 (S-B12-TC2) is therefore more accurate and is the primary way to determine vitamin B12 deficiency, as it does not take into account the inactive vitamin bound to haptocorrin. However, the vitamin bound to haptocorrin may mask deficiency.

Vitamin B12 deficiency is considered confirmed when the active vitamin B12 level in the serum is below 20 pmol/L. Deficiency is usually ruled out when the level exceeds 50 pmol/L. Within the gray area in between, vitamin B12 deficiency can be ruled out by measuring the metabolites that accumulate during deficiency: homocysteine (HCY) or methylmalonate (MMA). The latter is more specific but also more expensive to test. The increased metabolite level is indicative of active vitamin B12 deficiency. However, even test results in excess of 50 pmol/L do not necessarily mean that the vitamin B12 intake and levels are optimal for health. Some scientists and clinicians suggest 100 pmol/L as the target level. When the level exceeds the study method limits, the value is often given as > 128 pmol/L.

Finnish reference range (S-B12-TC2):

• Less than 35 pmol/L: Deficient
• 35–50 pmol/L: Borderline
• 50–128 pmol/L: Normal
• More than 128 pmol/L: Optimal

Clear vitamin B12 deficiency is ruled out when the level exceeds 50 pmol/L. However, the optimal level is 128 pmol/L or above.