Biomarkers and proposed reference ranges of vitamin B12 status, a summary:
Plasma/serum B12: above 221 (pmol/L);
Plasma holo-transcobalamin (holo-TC): above 40 (pmol/L);
Plasma/serum methylmalonic acid (MMA) with normal creatinine: less than 210 (nmol/L) (or less than 270 nmol/L in Strohle, 2019);
Plasma homocysteine: less than 12 (umol/L, “micro mole per litre”).