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heme.b12-folate-deficiency-anemia.core.v1PRODUCTION
heme.b12-folate-deficiency-anemia.core.v1

Vitamin B12 & Folate Deficiency Anaemia (megaloblastic; pernicious anaemia + SCD)

hematologychronicadultgeriatricpregnancy
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12/12 authored

Canonical 12-phase frame with authored status for this dossier.

Current phase

Frame

Detailed

Establish macrocytic/megaloblastic anaemia frame — partition B12 vs folate vs both vs non-megaloblastic macrocytosis; flag pernicious anaemia and subacute combined degeneration as the high-stakes phenotypes (BSH Devalia 2014 PMID 24942828)

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Megaloblastic frame and candidate aetiology assigned

Patient inputs (11)

Geriatric food-bound malabsorption + dementia-mimic routing; pregnancy folate demand (BSH Devalia 2014 PMID 24942828)

Periconceptional folate 400 mcg vs 4–5 mg if prior NTD; B12 in vegan pregnancy (MRC Vitamin Study PMID 1677062)

MCV >100 fL = macrocytosis; pancytopenia signals severe megaloblastosis vs leukaemia mimic (BSH Devalia 2014 PMID 24942828)

First-line B12; low sensitivity/specificity → reflex MMA + homocysteine (Stabler NEJM 2013 PMID 23301732; PMID 32089757)

Folate deficiency partition; NEVER replace folate alone if B12 unmeasured (SCD risk) (BSH Devalia 2014 PMID 24942828)

Elevated MMA + Hcy = B12; isolated Hcy = folate; both metabolites discriminate functional deficiency (N2O) when B12 normal (PMID 34427020; PMID 32089757)

Atrophic/pernicious gastritis, bariatric, ileal resection/Crohn, pancreatic insufficiency, fish tapeworm — determines lifelong vs short-course Rx (BSH Devalia 2014 PMID 24942828)

Metformin/PPI/H2RA (B12); methotrexate/trimethoprim/phenytoin (folate); N2O (functional B12) — deprescribe-or-monitor trigger (Aroda DPPOS PMID 26900641)

Low retic in megaloblastosis; reticulocytosis day 5–7 confirms repletion response (Stabler NEJM 2013 PMID 23301732)

Hypokalaemia during rapid haematologic repletion — monitor K+ (BSH Devalia 2014 PMID 24942828)

Paraesthesia/ataxia/dorsal-column/optic atrophy/cognitive change — may be irreversible if delayed; treat empirically if high suspicion (Stabler NEJM 2013 PMID 23301732)

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Severity triggers (6)

6 need judgement
  • informationallife_threateningfolate_alone_in_unmeasured_b12
    Folate replacement initiated/planned while B12 status unmeasured or B12 deficiency unexcluded (BSH Devalia 2014 PMID 24942828)
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalseveresevere_symptomatic_anaemia
    Hb critically low with cardiac symptoms (angina, syncope, high-output failure) from megaloblastic anaemia (BSH Devalia 2014 PMID 24942828)
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalsevereneurologic_involvement_scd
    Paraesthesia / gait ataxia / dorsal-column signs / optic atrophy / cognitive change = subacute combined degeneration (Stabler NEJM 2013 PMID 23301732)
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalseverepancytopenia_leukaemia_mimic
    Pancytopenia with profound macrocytosis + hypersegmented neutrophils + very high LDH mimicking acute leukaemia / MDS (BSH Devalia 2014 PMID 24942828)
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalseveren2o_myeloneuropathy
    Recreational nitrous-oxide use + myeloneuropathy with NORMAL serum B12 but elevated MMA/homocysteine ± dorsal-column MRI signal (Pract Neurol case series PMID 34427020)
    Trigger could not be auto-evaluated — needs clinician judgement.
  • informationalmoderatehypokalaemia_on_replacement
    Falling serum K+ during early rapid haematologic repletion (BSH Devalia 2014 PMID 24942828)
    Trigger could not be auto-evaluated — needs clinician judgement.

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Recommended regimen

B12 / folate repletion (route- and aetiology-driven)
axis: b12_folate_repletion
Selected axis "B12 / folate repletion (route- and aetiology-driven)" by default fallback (first axis)
  • hydroxocobalamin
    first line
    cobalamin (vitamin B12)
    1 mg • IM • alternate days for 1–2 weeks (until no further neuro improvement), then 1 mg every 2–3 months
    triggers: confirmed_b12_deficiency, neurologic_involvement_or_SCD, pernicious_anaemia, severe_symptomatic_anaemia, malabsorption
    BSH Devalia 2014 (PMID 24942828) — parenteral loading then lifelong maintenance for pernicious anaemia / irreversible malabsorption; preferred when neurologic involvement present
    rxcui 5514
  • cyanocobalamin
    first line
    cobalamin (vitamin B12)
    1000–2000 mcg • PO • once daily
    triggers: confirmed_b12_deficiency, dietary_or_reversible_cause, no_severe_neuro_disease, patient_preference_or_anticoagulated
    Cochrane Wang 2018 (PMID 29543316) — high-dose oral via passive absorption non-inferior to IM for short-term haematologic + neurologic response in many causes; also IM 1 mg form for parenteral course
    rxcui 11248
  • folic acid
    first line
    folate
    1–5 mg • PO • once daily for ~4 months (lifelong if chronic haemolysis / malabsorption)
    triggers: confirmed_folate_deficiency_with_B12_status_known, pregnancy_periconception, chronic_haemolysis
    BSH Devalia 2014 (PMID 24942828) — folate 1–5 mg/day; MRC Vitamin Study (PMID 1677062) periconceptional 400 mcg (4–5 mg if prior NTD). NEVER folate-alone if B12 unmeasured (SCD risk)
    rxcui 4511
  • leucovorin (folinic acid)
    second line
    reduced folate
    per indication • PO/IV • per protocol
    triggers: methotrexate_or_dihydrofolate_reductase_inhibitor_induced_folate_deficiency, trimethoprim_antifolate_toxicity
    Bypasses DHFR block — used when deficiency is driven by antifolate (methotrexate / trimethoprim / pyrimethamine) rather than dietary/absorptive folate lack
    rxcui 6313
  • methylcobalamin
    add on
    cobalamin (vitamin B12)
    500–1000 mcg • PO • once daily
    triggers: patient_preference_active_form, adjunct_in_n2o_myeloneuropathy
    Active cobalamin form; adjunct/alternative oral option (incl. N2O functional deficiency where methylation cycle is blocked) — evidence base weaker than hydroxocobalamin/cyanocobalamin
    rxcui 29421
  • discontinue / deprescribe offending agent + dietary correction + treat underlying
    add on
    cause-directed (non-pharmacologic)
    triggers: metformin_or_PPI_associated, N2O_use, vegan_diet, bariatric_or_ileal_resection, crohn_disease, pancreatic_insufficiency
    Stop N2O; deprescribe-or-monitor metformin/PPI (Aroda DPPOS PMID 26900641; de Jager HOME PMID 20488910); B12-fortified diet/supplements for vegans; lifelong supplementation post-bariatric/ileal resection; route Crohn to gi.crohns.core.v1
  • packed red blood cell transfusion
    rescue
    blood product
    triggers: severe_symptomatic_anaemia_with_cardiac_compromise
    Rarely needed; reserved for life-threatening symptomatic anaemia — transfuse cautiously (slow, small volume) given high-output state and risk of fluid overload (BSH Devalia 2014 PMID 24942828)

outpatient playbook — drug actions (4)

  1. 1. hydroxocobalamin
    rxcui 5514
    1 mg • IM • alternate days x1–2 wk then q2–3 months
    trigger: Confirmed B12 deficiency with pernicious anaemia / malabsorption / neuro involvement
    BSH Devalia 2014 — parenteral loading then lifelong maintenance (PMID 24942828)
  2. 2. cyanocobalamin (oral)
    rxcui 11248
    1000–2000 mcg • PO • daily
    trigger: Dietary/reversible B12 deficiency without severe neuro disease
    Cochrane Wang 2018 — high-dose oral non-inferior short term (PMID 29543316)
  3. 3. folic acid
    rxcui 4511
    1–5 mg (400 mcg–4 mg periconception) • PO • daily x ~4 months
    trigger: Folate deficiency with B12 status established; pregnancy/preconception
    BSH Devalia 2014 + MRC Vitamin Study (PMID 24942828; PMID 1677062) — only after B12 measured/replaced
  4. 4. leucovorin (folinic acid)
    rxcui 6313
    per indication • PO/IV • per protocol
    trigger: Antifolate-induced (methotrexate/trimethoprim) deficiency
    Bypasses DHFR block

Auto-drafted A&P note

outpatient

Subjective

- Possible entry pathways: Macrocytosis MCV >100 fL on CBC (BSH Devalia 2014 PMID 24942828); Low / borderline serum B12 (<148 pmol/L definite; 148–221 borderline) (Stabler NEJM 2013 PMID 23301732); Low serum / red-cell folate (BSH Devalia 2014 PMID 24942828).

Objective

- No vitals, labs, or imaging entered for this encounter.

Assessment

**Vitamin B12 & Folate Deficiency Anaemia (megaloblastic; pernicious anaemia + SCD)** (heme.b12-folate-deficiency-anemia.core.v1).
Phenotype framing: Terminal aetiologies: pernicious anaemia (autoimmune atrophic gastritis); dietary B12 (vegan); B12 malabsorption (atrophic gastritis, bariatric, ileal resection/Crohn, pancreatic insufficiency, Diphyllobothrium); drug-induced (metformin/PPI/H2RA → B12; methotrexate/trimethoprim/phenytoin → folate); N2O functional B12 deficiency; folate — dietary, alcohol, increased demand (pregnancy/haemolysis), MTHFR; combined B12+folate; non-megaloblastic macrocytosis (alcohol, liver disease, hypothyroidism, MDS, reticulocytosis) (BSH Devalia 2014 PMID 24942828)
Scope: Establish macrocytic/megaloblastic anaemia frame — partition B12 vs folate vs both vs non-megaloblastic macrocytosis; flag pernicious anaemia and subacute combined degeneration as the high-stakes phenotypes (BSH Devalia 2014 PMID 24942828)

No severity triggers fired against current inputs.

Plan

Regimen axis: **B12 / folate repletion (route- and aetiology-driven)**.
1. hydroxocobalamin 1 mg IM alternate days for 1–2 weeks (until no further neuro improvement), then 1 mg every 2–3 months (cobalamin (vitamin B12), first line) — BSH Devalia 2014 (PMID 24942828) — parenteral loading then lifelong maintenance for pernicious anaemia / irreversible malabsorption; preferred when neurologic involvement present
2. cyanocobalamin 1000–2000 mcg PO once daily (cobalamin (vitamin B12), first line) — Cochrane Wang 2018 (PMID 29543316) — high-dose oral via passive absorption non-inferior to IM for short-term haematologic + neurologic response in many causes; also IM 1 mg form for parenteral course
3. folic acid 1–5 mg PO once daily for ~4 months (lifelong if chronic haemolysis / malabsorption) (folate, first line) — BSH Devalia 2014 (PMID 24942828) — folate 1–5 mg/day; MRC Vitamin Study (PMID 1677062) periconceptional 400 mcg (4–5 mg if prior NTD). NEVER folate-alone if B12 unmeasured (SCD risk)
4. leucovorin (folinic acid) per indication PO/IV per protocol (reduced folate, second line) — Bypasses DHFR block — used when deficiency is driven by antifolate (methotrexate / trimethoprim / pyrimethamine) rather than dietary/absorptive folate lack
5. methylcobalamin 500–1000 mcg PO once daily (cobalamin (vitamin B12), add on) — Active cobalamin form; adjunct/alternative oral option (incl. N2O functional deficiency where methylation cycle is blocked) — evidence base weaker than hydroxocobalamin/cyanocobalamin
6. discontinue / deprescribe offending agent + dietary correction + treat underlying (cause-directed (non-pharmacologic), add on) — Stop N2O; deprescribe-or-monitor metformin/PPI (Aroda DPPOS PMID 26900641; de Jager HOME PMID 20488910); B12-fortified diet/supplements for vegans; lifelong supplementation post-bariatric/ileal resection; route Crohn to gi.crohns.core.v1
7. packed red blood cell transfusion (blood product, rescue) — Rarely needed; reserved for life-threatening symptomatic anaemia — transfuse cautiously (slow, small volume) given high-output state and risk of fluid overload (BSH Devalia 2014 PMID 24942828)

Setting playbook (outpatient) — Confirm and partition B12 vs folate deficiency, identify aetiology (esp. pernicious anaemia + reversible/drug causes), replete safely (B12 before/with folate), and arrange lifelong vs short-course follow-up (BSH Devalia 2014 PMID 24942828)
8. hydroxocobalamin 1 mg IM alternate days x1–2 wk then q2–3 months — Confirmed B12 deficiency with pernicious anaemia / malabsorption / neuro involvement (BSH Devalia 2014 — parenteral loading then lifelong maintenance (PMID 24942828))
9. cyanocobalamin (oral) 1000–2000 mcg PO daily — Dietary/reversible B12 deficiency without severe neuro disease (Cochrane Wang 2018 — high-dose oral non-inferior short term (PMID 29543316))
10. folic acid 1–5 mg (400 mcg–4 mg periconception) PO daily x ~4 months — Folate deficiency with B12 status established; pregnancy/preconception (BSH Devalia 2014 + MRC Vitamin Study (PMID 24942828; PMID 1677062) — only after B12 measured/replaced)
11. leucovorin (folinic acid) per indication PO/IV per protocol — Antifolate-induced (methotrexate/trimethoprim) deficiency (Bypasses DHFR block)

Non-pharmacologic actions:
- Deprescribe or monitor metformin / PPI; stop nitrous-oxide use (Aroda DPPOS PMID 26900641)
- B12-fortified diet / supplements for vegans; folate-rich diet counselling
- Refer Crohn / IBD malabsorption to gi.crohns.core.v1
- Endoscopic referral for autoimmune atrophic gastritis surveillance (gastric cancer/carcinoid risk) (PMID 23216458)
- Periconception folate counselling for women planning pregnancy (MRC Vitamin Study PMID 1677062)

AVOID / contraindication checks:
- NEVER replace folate alone when B12 status is unmeasured or B12 deficiency unexcluded — risk of precipitating/worsening subacute combined degeneration (BSH Devalia 2014 PMID 24942828; Stabler NEJM 2013 PMID 23301732)
- Monitor serum potassium during early rapid repletion — hypokalaemia from brisk erythropoiesis (BSH Devalia 2014 PMID 24942828)
- Treat empirically with B12 if neurologic features and high suspicion — do NOT delay for confirmatory MMA (irreversibility risk)
- In N2O induced functional deficiency serum B12 may be NORMAL — use MMA/homocysteine + clinical/MRI, treat with high dose B12 ± methionine and stop N2O (PMID 34427020)
- Pernicious anaemia / irreversible malabsorption requires LIFELONG maintenance — do not stop after loading course (BSH Devalia 2014 PMID 24942828)

Monitoring

Regimen monitoring:
- Reticulocyte count day 5 to 7 (response confirmation)
- CBC MCV at 2wk and 8wk (MCV normalises ~8 wk)
- Serum K during early repletion (hypokalaemia)
- Platelet count (rebound thrombocytosis)
- Ferritin iron studies if MCV not normalising (unmasked iron deficiency)
- Neuro reassessment for SCD recovery

Setting (outpatient) monitoring:
- Reticulocyte count day 5–7 (BSH Devalia 2014 PMID 24942828)
- CBC + MCV at 2 and 8 weeks (PMID 24942828)
- Serum K+ during early repletion (hypokalaemia) (PMID 24942828)
- Annual FBC + thyroid in pernicious anaemia (autoimmune association)

Follow-up plan: Pernicious anaemia: lifelong B12 + endoscopic gastric surveillance per local pathway (gastric adenocarcinoma/carcinoid risk, ~7× RR, PMID 23216458); annual FBC + thyroid (autoimmune association — panel.thyroid). Drug-induced: periodic B12 on continued metformin/PPI (Aroda DPPOS PMID 26900641). Pregnancy: periconceptional folate 400 mcg standard, 4–5 mg if prior NTD/high-risk, continue through 12 wk; B12 in vegans/post-bariatric; infants of B12-deficient mothers screened. Geriatric: re-screen cognition if dementia-mimic. Bariatric/ileal resection: lifelong supplementation + monitoring (BSH Devalia 2014 PMID 24942828; MRC Vitamin Study PMID 1677062)
- Close-out criterion: Lifelong/interval plan + surveillance documented

Monitoring phase: Reticulocyte count day 5–7 (response confirmation); repeat CBC + MCV at ~2 and ~8 weeks (MCV normalises ~8 wk); serum K+ during early rapid repletion (hypokalaemia risk); platelet count (rebound thrombocytosis); B12/folate not routinely re-checked post-loading on adequate replacement; ferritin/iron studies if MCV fails to normalise (unmasked iron deficiency); neuro re-assessment for SCD recovery (BSH Devalia 2014 PMID 24942828)

Disposition

Current setting: outpatient — Confirm and partition B12 vs folate deficiency, identify aetiology (esp. pernicious anaemia + reversible/drug causes), replete safely (B12 before/with folate), and arrange lifelong vs short-course follow-up (BSH Devalia 2014 PMID 24942828)

Disposition criteria:
- Continue outpatient repletion + scheduled follow-up if stable (BSH Devalia 2014 PMID 24942828)
- Lifelong maintenance + surveillance plan for pernicious anaemia / irreversible malabsorption (PMID 24942828)
- Discharge from engine once Hb/MCV normalised and stable replacement plan documented

Escalation triggers (move to higher acuity):
- Progressive neurologic deficit / new dorsal-column signs → urgent parenteral B12 + neuro escalation (Stabler NEJM 2013 PMID 23301732)
- Severe symptomatic anaemia (angina, syncope, high-output failure) → inpatient (BSH Devalia 2014 PMID 24942828)
- Pancytopenia or no reticulocyte response → haematology / marrow to exclude MDS/leukaemia (PMID 24942828)
- Symptomatic hypokalaemia on replacement → inpatient correction (PMID 24942828)

Earlier-Return Triggers

Return-precaution thresholds (watch for):
- [LIFE_THREATENING] Folate replacement initiated/planned while B12 status unmeasured or B12 deficiency unexcluded (BSH Devalia 2014 PMID 24942828)
- [SEVERE] Hb critically low with cardiac symptoms (angina, syncope, high-output failure) from megaloblastic anaemia (BSH Devalia 2014 PMID 24942828)
- [SEVERE] Paraesthesia / gait ataxia / dorsal-column signs / optic atrophy / cognitive change = subacute combined degeneration (Stabler NEJM 2013 PMID 23301732)

Citations

- BSH/BCSH Guidelines for the diagnosis & treatment of cobalamin & folate disorders (Devalia, BJH 2014) + Stabler NEJM 2013 clinical practice review + Cochrane oral-vs-IM B12 (Wang 2018) + ACOG/MRC periconceptional folate + N2O-myeloneuropathy literature [PMID:24942828](https://pubmed.ncbi.nlm.nih.gov/24942828/)
- Cited evidence (PMID 23301732) [PMID:23301732](https://pubmed.ncbi.nlm.nih.gov/23301732/)
- Cited evidence (PMID 29543316) [PMID:29543316](https://pubmed.ncbi.nlm.nih.gov/29543316/)
- Cited evidence (PMID 34427020) [PMID:34427020](https://pubmed.ncbi.nlm.nih.gov/34427020/)
- Cited evidence (PMID 26900641) [PMID:26900641](https://pubmed.ncbi.nlm.nih.gov/26900641/)

Last reconciled with current guidelines: 2026-05-16.
References
  • BSH/BCSH Guidelines for the diagnosis & treatment of cobalamin & folate disorders (Devalia, BJH 2014) + Stabler NEJM 2013 clinical practice review + Cochrane oral-vs-IM B12 (Wang 2018) + ACOG/MRC periconceptional folate + N2O-myeloneuropathy literature — PMID:24942828
  • Cited evidence (PMID 23301732) — PMID:23301732
  • Cited evidence (PMID 29543316) — PMID:29543316
  • Cited evidence (PMID 34427020) — PMID:34427020
  • Cited evidence (PMID 26900641) — PMID:26900641