⚕️ Educational content only. This article is intended for biomedical science students and those interested in laboratory medicine. It is not medical advice. Consult a healthcare professional for personal health concerns.
Vitamin B12 (cobalamin) and folate (vitamin B9) are essential water-soluble vitamins required for DNA synthesis, red blood cell production, and neurological function. Both are measured via routine blood tests and deficiency in either can lead to megaloblastic anaemia. Understanding the biochemistry and laboratory approach helps interpret results accurately.
Key Takeaways
- Both B12 and folate are required for thymidine synthesis; deficiency impairs DNA replication and causes megaloblastic anaemia.
- B12 deficiency also causes subacute combined degeneration of the spinal cord; folate deficiency does not.
- Serum B12 levels can be normal in functional B12 deficiency; methylmalonic acid (MMA) and homocysteine are more sensitive markers.
- Folate deficiency is commonly dietary; B12 deficiency is often due to malabsorption (pernicious anaemia, gastrectomy, ileal disease).
Why B12 and Folate Matter
Both vitamins are co-factors in one-carbon metabolism. Folate (as 5,10-methylenetetrahydrofolate) is required for thymidylate synthesis. B12 is needed to regenerate tetrahydrofolate from 5-methyltetrahydrofolate. When either is deficient, DNA synthesis is impaired, and cells unable to divide properly — particularly rapidly dividing cells like erythroblasts — become enlarged. This produces macrocytic, megaloblastic red cells and hypersegmented neutrophils on the blood film.
Causes of Deficiency
Vitamin B12 Deficiency
The commonest cause in developed countries is pernicious anaemia — an autoimmune condition where antibodies target gastric parietal cells or intrinsic factor (IF), preventing B12 absorption in the terminal ileum. Other causes include: gastrectomy or gastric bypass (loss of parietal cells), ileal resection or Crohn’s disease (loss of absorption site), strict veganism (B12 is found only in animal products), metformin use (reduces B12 absorption), and bacterial overgrowth (competing for B12).
Folate Deficiency
Folate deficiency is most commonly dietary (poor intake of green vegetables, legumes, and fortified foods). It is also seen in: pregnancy and haemolytic anaemia (increased demand), malabsorption (coeliac disease, Crohn’s), alcohol excess (impairs absorption and increases excretion), and drugs such as methotrexate, trimethoprim, and phenytoin (folate antagonists).
Laboratory Tests
Serum B12: The first-line test. Normal range is approximately 150–700 pmol/L (varies by lab). Low levels suggest deficiency, but borderline results are common. Serum B12 can be normal in functional B12 deficiency, particularly in patients with high serum haptocorrin. Red cell folate reflects longer-term folate status (2–3 months) compared to serum folate (recent intake). Methylmalonic acid (MMA) is elevated specifically in B12 deficiency (it accumulates when the B12-dependent enzyme methylmalonyl-CoA mutase cannot function). Homocysteine is elevated in both B12 and folate deficiency. These metabolite tests are useful when B12/folate results are borderline or clinically inconsistent.
Blood Film Findings
Megaloblastic anaemia produces macrocytic red cells (MCV >100 fL), oval macrocytes, and hypersegmented neutrophils (≥5 lobes in >5% of neutrophils). These findings indicate impaired DNA synthesis regardless of the underlying cause. Combined with low B12 or folate, they confirm megaloblastic anaemia. A bone marrow examination may show megaloblastic change in the erythroid lineage if further evaluation is needed.
The Importance of Testing Both
Treating folate deficiency with folic acid supplementation when undiagnosed B12 deficiency coexists can correct the anaemia but unmask or worsen the neurological complications of B12 deficiency. Therefore, B12 levels should always be checked before starting folate replacement in patients with megaloblastic anaemia.
References
- NHS. Vitamin B12 or folate deficiency anaemia. nhs.uk
- NIH Office of Dietary Supplements. Vitamin B12 Fact Sheet. ods.od.nih.gov
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149-160.