When should B12 be retested?

B12 retesting should be guided by clinical context, not routine screening. Typical scenarios: 1) After starting supplementation: recheck in 2–3 months to confirm normalization. 2) In conditions with ongoing malabsorption (pernicious anemia, gastric bypass, long-term PPI use): annual monitoring. 3) In vegans/vegetarians not supplementing: every 1–2 years, as stores deplete slowly. 4) If neurological symptoms (neuropathy, gait disturbance) or macrocytic anemia appear: retest immediately even if prior levels were normal. Important: Serum B12 alone can be misleading – when borderline or symptoms are present, measure holotranscobalamin (holo-TC) and methylmalonic acid (MMA). Evidence is strong (clinical guidelines from DGE, NICE, etc.). Caveat: Do not self-test without medical oversight if deficiency is suspected.