For normal values: B12 immunoassays may rarely be falsely normal in patients with deficiency. If the clinical suspicion remains high (e.g. unexplained significant macrocytic anemia), consider ordering serum methymalonic acid).
For low values: B12 immunoassays have suboptimal diagnostic specificity. Although a trial of B12 supplementation may be reasonable, confirmatory testing with serum methylmalonic acid is recommended. Keep in mind that modest elevations in MMA are common with even mild renal insufficiency and in older adults due to decreased renal clearance. True deficiency usually has very high MMA values.
a. B-12, Vitamin
b. Vit B-12