Folate Deficiency Anemia

Risk factors

• Inadequate intake: 'tea and toast' diet, elderly, alcoholism (combined poor intake + impaired absorption + increased turnover)
• Increased demand: pregnancy, lactation, hemolytic anemia (sickle cell, thalassemia), exfoliative skin disease, hemodialysis
• Malabsorption: celiac disease, tropical sprue, short bowel, jejunal disease
• Drugs: methotrexate, trimethoprim, phenytoin, sulfasalazine, triamterene, phenobarbital, oral contraceptives (mild)
• Goat milk diet in infants (folate-poor)

Pathophysiology

Folate (5-methyl-THF) donates a methyl group to homocysteine to form methionine (B12-dependent step) and contributes single-carbon units for de novo synthesis of purines and dTMP. Deficiency halts DNA synthesis while RNA and cytoplasmic maturation continue, yielding large nucleated megaloblasts in the marrow and ineffective erythropoiesis with oval macrocytes and hypersegmented neutrophils in the periphery. Unlike B12, folate is not required for myelin maintenance.

Clinical presentation

Differential diagnosis

• Vitamin B12 deficiency — Same hematologic picture PLUS neurologic features; methylmalonic acid elevated (normal in folate deficiency)
• Alcohol-related macrocytosis — Direct marrow toxicity; round macrocytes; often coexists with folate deficiency
• Myelodysplastic syndrome — Elderly, persistent cytopenias, dysplasia, blasts; does not respond to folate
• Drug-induced antifolate — Methotrexate, trimethoprim — routine folic acid reduces low-dose methotrexate toxicity; significant toxicity or high-dose methotrexate needs leucovorin (folinic acid) rescue
• Hypothyroidism, liver disease — Macrocytosis without megaloblastic features

Diagnostic workup

Diagnostic algorithm

flowchart TD
  A[Macrocytic anemia<br/>MCV >100] --> B[Peripheral smear]
  B --> C{Megaloblastic?<br/>Oval macrocytes<br/>Hypersegmented PMNs}
  C -->|No| D[Non-megaloblastic<br/>Liver dz, hypothyroid,<br/>EtOH, MDS, drugs]
  C -->|Yes| E[Check B12 and folate]
  E --> F{Pattern}
  F -->|B12 low or<br/>borderline| G[Check MMA + homocysteine]
  G --> H[MMA elevated:<br/>B12 deficiency]
  F -->|Folate low,<br/>B12 normal| I[Check MMA + homocysteine]
  I --> J[MMA normal,<br/>Hcy elevated:<br/>Folate deficiency]
  H --> K[Treat with B12<br/>± folate]
  J --> L[Treat with folate<br/>after B12 confirmed normal]

Treatment

Complications

• Severe pancytopenia → bleeding, infection
• Hyperhomocysteinemia — increased thromboembolic and cardiovascular risk
• Neural tube defects (spina bifida, anencephaly) if deficient in early pregnancy
• Adverse pregnancy outcomes: preterm birth, low birth weight
• Masking B12 deficiency: folate replacement corrects megaloblastosis but allows B12-related neurologic damage to progress

PANCE pearls

• Body stores of folate last only ~3-4 months; B12 stores last 3-5 years. Folate deficiency develops far faster.
• Always measure B12 before starting folate — the most dangerous error is treating undiagnosed B12 deficiency with folate, which corrects the anemia but allows neurologic deterioration.
• US grain products have been folate-fortified since 1998, dramatically reducing both deficiency anemia and neural tube defects.
• Methotrexate and trimethoprim inhibit dihydrofolate reductase — with low-dose methotrexate (RA, psoriasis), routine folic acid 1 mg daily reduces antifolate toxicity, but high-dose methotrexate rescue or overdose requires folinic acid (leucovorin), which bypasses the DHFR block.
• Folate deficiency does NOT cause neurologic disease. New paresthesias or ataxia in a megaloblastic anemia patient point to B12 deficiency until proven otherwise.
• Phenytoin and other antiepileptics impair folate absorption — anyone of childbearing potential on these drugs should take at least 0.4 mg/day folic acid before conception and throughout pregnancy (AAN/AES/SMFM 2024); 4 mg/day is for a prior NTD-affected pregnancy.

References

• BSH 2014 — British Society for Haematology Guidelines for the diagnosis and treatment of cobalamin and folate disorders (Devalia et al., Br J Haematol 2014)
• USPSTF 2023 — Folic Acid Supplementation to Prevent Neural Tube Defects: US Preventive Services Task Force Reaffirmation Recommendation Statement (JAMA 2023;330(5):454-459)
• ACOG PB 187 — ACOG Practice Bulletin No. 187: Neural Tube Defects (Obstet Gynecol 2017;130(6):e279-e290)
• Green & Datta Mitra — Megaloblastic Anemias: Nutritional and Other Causes (Green & Datta Mitra, Med Clin North Am 2017)
• AAN/AES/SMFM 2024 — Teratogenesis, Perinatal, and Neurodevelopmental Outcomes After In Utero Exposure to Antiseizure Medication: Practice Guideline From the AAN, AES, and SMFM (Pack AM et al., Neurology 2024;102(11):e209279)