assessment, investigation and diagnosis of B12 deficiency

Last edited 05/2022 and last reviewed 06/2023

  • Clinical.
  • Diagnosis of megaloblastic anaemia:
    • FBC - macrocytic anaemia; a thrombocytopaenia may occur secondary to ineffective megakaryopoiesis
    • blood film - macrocytosis, hypersegmented neutrophils
    • bone marrow - megaloblastic, erythropoiesis, giant metamyelocyte
  • Measurement of B12 / folate levels - in B12 deficiency red cell folate levels are low (B12 required in synthesis) but serum folate is generally normal or high (1).
  • Measurement of B12 absorption - Schilling test - very rarely undertaken now (3).
  • Anti-intrinsic factor (anti-IF) antibodies in serum:
    • positive in about 50% of patients with pernicious anaemia (1)
    • the presence of intrinsic factor (IF) is diagnostic of pernicious anaemia but negative intrinsic factor antibodies does not exclude pernicious anaemia (due to the test’s low sensitivity (50-60%)
    • highly specific but not very sensitive (2)
  • Anti-gastric parietal cell (GPC) antibodies in serum:
    • seen in 95% of cases of pernicious anaemia and, although there is an overlap with other autoimmune diseases and with normal individuals, a negative result makes pernicious anaemia unlikely (1)
    • reasonably sensitive but not as specific as anti-IF antibodies
      • note a review stated testing for anti-gastric parietal cell antibodies is not recommended because of the variable specificity of 50-100% (3)
  • A positive anti-GPC and/or anti-IF antibody test does need repeating (2).
  • Other investigations to help define the cause of the vitamin B12 deficiency:
    • thyroid function tests and anti-thyroid antibodies
    • test for coeliac disease
      • tissue transglutaminase (tTG)
    • tests for generalised malabsorption (if symptoms are suggestive) - faecal tests are generally only requested by a gastroenterologist/after gastroenterological advice
      • serum
        • calcium and vitamin D
        • folate
        • ferritin
      • faecal
        • fats
        • elastase
  • Urinary methylmalonyl CoA urinary excretion is increased in B12 deficiency - B12 is the co-enzyme in the conversion reaction of methylmalonyl CoA to succinyl CoA.


  • Clinical picture is the most important factor in assessing the results of the serum vitamin B12. Definitive cut off points for clinical and subclinical deficiency are not possible (1).
    • the test measures total, not metabolically active vitamin B12
    • levels are not easily correlated with clinical symptoms, although patients with vitamin B12 levels <100ng/L almost always have clinical or metabolic evidence of vitamin B12 deficiency, and <150ng/l usually do
    • in most patients with clinically significant vitamin B12 deficiency, the serum level is below 200ng/L but clinically significant vitamin B12 deficiency may be present even when levels are in the normal range, especially in elderly patients (1)
    • about a third of patients with B12 deficiency may not have macrocytosis (3)