Assessment, investigation and diagnosis of B12 deficiency

• 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.

Notes:

• 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)

Reference:

• (1) NHS Wiltshire CCG. Investigation and treatment of Vitamin B12 (cobalamin) deficiency in primary care (accessed 13/05/2022).
• (2) Royal United Hospital Bath NHS Trust. Guidelines for the Investigation & Management of vitamin B12 deficiency (accessed 13/05/2022).
• (3) Mohamed M, Thio J, Thomas RS, Phillips J. Pernicious anaemia. BMJ. 2020 Apr 24;369:m1319. doi: 10.1136/bmj.m1319. PMID: 32332011.