B12 deficiency
Vitamin B12 is a critical water-soluble vitamin, essential for normal neurological function and haematopoiesis. Despite the widespread availability of nutritious food, vitamin B12 deficiency remains relatively common due to its complex absorption pathway and various contributing clinical factors.
Biological Role of Vitamin B12
Vitamin B12 serves as a cofactor for two key enzymes:
- Methionine synthase – required for DNA synthesis and methylation.
- Methylmalonyl-CoA mutase – involved in fatty acid and amino acid metabolism.
Deficiency impairs these pathways, leading to:
- Impaired DNA synthesis → megaloblastic anaemia
- Neurological dysfunction → peripheral neuropathy, cognitive decline
- Systemic effects on multiple organ systems
- Neurologic Function:
- Peripheral Neuropathy: Vitamin B12 deficiency can lead to nerve damage, causing symptoms like tingling, numbness, and burning sensations in the hands and feet.
- Cognitive Impairment: Deficiency can result in memory loss, confusion, and cognitive decline.
- Gait Impairment: Patients may experience balance problems and difficulty walking.
- Red Blood Cell Production:
- Macrocytic Anemia: B12 deficiency causes the production of abnormally large and immature red blood cells.
- Pancytopenia: This condition can also lead to a decrease in all types of blood cells, including white blood cells and platelets.
- DNA Synthesis:
- B12-dependent Reactions:
- Methylmalonic Acid to Succinyl-CoA: B12 is essential for converting methylmalonic acid to succinyl-CoA, a critical component of the Krebs cycle.
- Homocysteine to Methionine: B12 helps convert homocysteine to methionine, which is vital for angiogenesis and other cellular functions.
- 5-Methyltetrahydrofolate to Tetrahydrofolate: This conversion is crucial for DNA synthesis and red blood cell production.
- B12-dependent Reactions:
Absorption, Transport, and Clinical Implications
1. Dietary Source and Requirement
- Vitamin B12 (cobalamin) is an essential nutrient that cannot be synthesised endogenously in humans.
- It must be obtained from animal-derived foods (e.g. meat, dairy, eggs) or fortified foods (e.g. cereals).
- Strict vegetarians and vegans are at risk of deficiency without supplementation.
2. Absorption Mechanism: A Multi-Step Process
A. Ingestion and Gastric Phase
- Protein-bound B12 is released in the stomach by pepsin and hydrochloric acid (HCl).
- It initially binds to haptocorrin (R-binder), secreted by salivary and gastric glands.
- Gastric acidity is crucial: PPIs and H2 blockers may impair this phase.
B. Duodenal Phase
- In the duodenum, pancreatic proteases degrade haptocorrin, freeing B12.
- Free B12 then binds to intrinsic factor (IF), a glycoprotein secreted by gastric parietal cells.
C. Ileal Absorption
- The B12–IF complex travels to the terminal ileum, where it binds to specific cubilin receptors and is absorbed via receptor-mediated endocytosis.
- This is the primary physiological pathway for B12 absorption.
3. Alternate Absorption Pathway
A passive diffusion pathway, independent of IF and the ileum, allows ~1% of an oral megadose to be absorbed.
- This forms the basis of oral high-dose B12 therapy in deficiency states.
Post-Absorptive Transport
Once absorbed, vitamin B12 enters the bloodstream and binds to two transport proteins:
| Protein | Proportion | Function |
|---|---|---|
| Haptocorrin | ~80% | Biologically inactive; its role is unclear. Elevated in liver disease, malignancies, and reduced in pregnancy. |
| Transcobalamin II | ~20% | Binds to form holotranscobalamin, the biologically active form taken up by cells. |
Storage and Reserve
- Liver stores account for 2–5 mg of vitamin B12, sufficient for 3–5 years.
- This explains the delayed onset of deficiency after absorption is impaired.
- Patients often have a decade of iron-deficiency–related symptoms (e.g. restless legs, pica) before macrocytic anaemia emerges.
Clinical Manifestations of Vitamin B12 Deficiency
Haematological
- Megaloblastic anaemia: Macrocytic anaemia with hypersegmented neutrophils
- Pancytopenia in severe cases
- Pernicious anaemia: An autoimmune cause of B12 deficiency with gastric atrophy
Neurological
- Paresthesias, numbness, burning sensations
- Peripheral neuropathy
- Subacute combined degeneration:
- Involves posterior columns and corticospinal tracts
- Proprioceptive loss, ataxia, hyporeflexia, spasticity, incontinence
Neuropsychiatric
- Depression
- Cognitive impairment, dementia
- Personality change, psychosis
- Potential association with Alzheimer’s disease in observational studies
Cardiovascular
- Elevated homocysteine levels may increase risk of atherothrombosis, MI, and stroke (though causality remains debated)
Maternal and Infant Outcomes
- Maternal B12 deficiency:
- Neural tube defects
- Intrauterine growth restriction
- Developmental delay, failure to thrive, hypotonia, ataxia in infants
Pernicious Anaemia
| Feature | Description |
|---|---|
| Cause | Autoimmune destruction of parietal cells → ↓IF |
| Autoantibodies | Anti-intrinsic factor anti-parietal cell |
| Associations | T1DM Hashimoto’s Vitiligo Autoimmune gastritis Addison disease |
| Epidemiology | Common in elderly (>60 years) |
| Onset | Gradual over 2–5 years Often starts in early–mid adulthood (20s–40s) but is silent Clinical pernicious anaemia – Peak incidence 60–80 years; uncommon <40 y |
| Features | Megaloblastic anaemia neurological symptoms glossitis |
Differential Diagnosis of Vitamin B12 Deficiency
1. Impaired Intrinsic Factor (IF) Production
- Atrophic gastritis
- Age-related mucosal atrophy
- Helicobacter pylori infection
- Associated with type 1 diabetes
- More common in the elderly
- Pernicious anaemia
- Autoimmune destruction of gastric parietal cells
- Anti-intrinsic factor and anti-parietal cell antibodies
- Progression:
- Asymptomatic gastritis → appears up to 30 years prior
- Iron deficiency → precedes B12 deficiency by ~20 years
- Post-gastrectomy states
- Roux-en-Y gastric bypass
- Total or subtotal gastrectomy
- (deficiency may appear within 12–24 months because parietal cells are surgically bypassed or removed.)
2. Impaired Ileal Absorption
- Inflammatory or structural disease
- Crohn’s disease
- Ileal resection
- Infectious or infiltrative conditions
- Whipple’s disease
- Fish tapeworm (Diphyllobothrium latum) infestation
- Other causes
- Zollinger–Ellison syndrome (gastrinoma) – acid inactivation of pancreatic enzymes impairs B12 release from haptocorrin
3. Medication-Induced Malabsorption
- Metformin – impairs B12–IF complex uptake in the terminal ileum
- Proton pump inhibitors (PPIs) – reduce gastric acid, impairing B12 release from food
- H2 receptor antagonists – similar effect to PPIs
4. Genetic and Other Malabsorptive Causes
- Coeliac disease – mucosal damage to terminal ileum
- Congenital transcobalamin II deficiency – rare autosomal recessive disorder causing functional B12 deficiency
5. Inadequate Dietary Intake
- Vegan diets – B12 is absent from non-animal sources
- Elderly adults (>75 years) – reduced intake and hypochlorhydria, gastric atrophy
- Chronic alcoholism – poor intake and mucosal damage
- Psychiatric illness – dietary neglect
- Exclusively breastfed infants of vegan mothers – limited B12 supply from maternal milk
When to order B12 studies
Clinical features
- Megaloblastic or macrocytic anaemia, pancytopenia
- Peripheral neuropathy, dorsal-column signs, cognitive decline, dementia
- Unexplained fatigue, glossitis, infertility, depression
Conditions with high prevalence
- Malabsorption: coeliac disease, IBD, pancreatic insufficiency, post-bariatric surgery
- Dietary restriction: vegan/vegetarian diets, severe malnutrition, alcoholism
- Medications: metformin, PPIs, H2-blockers, nitrous-oxide exposure, colchicine
- Age > 65 y
Ix:
- FBC
- There is no single gold standard for diagnosing vitamin B12 deficiency.
- A combination of tests is often required, particularly when clinical suspicion is high despite normal serum B12.
1. Tests of Serum B12 Levels
a. Total Vitamin B12
- Common screening test but has limited sensitivity/specificity due to high proportion of inactive haptocorrin-bound B12.
- Reference intervals:
- Normal: >180 pmol/L
- Borderline: 150–180 pmol/L
- Deficient: <150 pmol/L
b. Holotranscobalamin (Active B12)
- Better correlates with tissue B12 status.
- Medicare rebate applicable if total B12 is low or borderline (per MBS guidelines).
- Reference intervals:
- Normal: >35 pmol/L
- Borderline: 30–35 pmol/L
- Deficient: <30 pmol/L
2. Tests of B12 Metabolic Function
Deficient B12 activity leads to accumulation of precursor metabolites:
a. Homocysteine
- Elevated in B12, folate, or B6 deficiency.
- Not specific to B12 deficiency.
b. Methylmalonic Acid (MMA)
- More specific than homocysteine.
- Elevated in B12 deficiency, but may be confounded by renal impairment or rare metabolic disorders.
- Note: This test incurs an out-of-pocket cost.
3. Autoantibody Testing
Indicated in suspected autoimmune aetiology of B12 deficiency:
- Anti-parietal cell antibodies
- Anti-intrinsic factor antibodies
Suggestive of pernicious anaemia if positive.
| Test | What it measures | Reference range† | Strengths / limitations |
|---|---|---|---|
| Total serum B12 | Holo-TC + holo-haptocorrin | > 180 pmol/L (deficient < 150; borderline 150–180) | Widely available; insensitive in early deficiency; elevated in liver/haemato-oncology states. |
| Holotranscobalamin (Active B12) | Bio-available fraction only | > 35 pmol/L (deficient < 30; borderline 30–35) | Better correlation with tissue stores; Medicare-rebate when total B12 low/borderline (automatically reflexed by many labs). |
| Methylmalonic acid (MMA) | Functional marker of methylmalonyl-CoA mutase | Age- & lab-specific (typically < 0.40 µmol/L) | ↑ with B12 deficiency; falsely ↑ in renal impairment, inherited metabolic disorders; out-of-pocket cost. |
| Total homocysteine | Functional marker of methionine synthase | < 15 µmol/L (fasting) | ↑ with B12, folate, B6 deficiency, hypothyroidism, renal impairment; non-specific. |
| Anti-intrinsic factor Ab | Autoimmune pernicious anaemia | Positive/negative | High specificity; low sensitivity (~50 %). |
| Anti-parietal cell Ab | Autoimmune gastritis | Positive/negative | High sensitivity; low specificity (false-positives in 7–10 % of healthy adults). |
Interpreting results
| Test Result | Interpretation |
|---|---|
| Normal total B12 & active B12 | B12 deficiency unlikely |
| Low active B12 (regardless of total B12) | Suggests functional deficiency |
| Normal B12 with strong clinical suspicion | Consider functional markers (MMA/homocysteine) or therapeutic trial |
- Total B12 ≥ 180 pmol/L AND
- Active B12 ≥ 35 pmol/L
Deficiency unlikely → seek alternative diagnosis.
- Active B12 ≥ 35 pmol/L
- Total B12 < 150 pmol/L OR
- Active B12 < 30 pmol/L
Deficiency very likely → treat; screen for cause (pernicious anaemia, malabsorption).
- Active B12 < 30 pmol/L
- Borderline zone
- (Total 150–180 pmol/L or Active 30–35 pmol/L)
Step-down testing: check MMA ± homocysteine.
If functional marker elevated → treat.
If markers normal but clinical suspicion remains → consider therapeutic trial (e.g., 1 mg IM hydroxocobalamin) and reassess symptoms/indices.
- (Total 150–180 pmol/L or Active 30–35 pmol/L)
- Normal labs but strong clinical suspicion
Early neurological manifestations may precede lab changes; initiate empirical therapy and monitor.
as of 1 July 2025 MBS Changes Mean for Providers (Vitamin B12 Testing)
🧪 New MBS Item for Initial Testing (66838)
- GPs can request Medicare-funded testing for total vitamin B12 and/or holotranscobalamin (active B12) under a new first-line test item (66838).
- Effective from: 1 July 2025.
- Benefit: Enables early identification of B12 deficiency using the biologically active form.
🔁 Follow-Up Testing (66839)
- If initial B12 result is abnormal or inconclusive, clinicians can request homocysteine and/or methylmalonic acid testing under item 66839 in the same episode of care.
🔄 Revised Frequency Restrictions
- Current 12-month restriction for item 66838 is reduced to 11 months.
- This same 11-month restriction will also apply to 66839.
🆘 More Frequent Testing for High-Risk Groups (66842)
- Item 66842 allows exceptions to the frequency rule based on clinical need.
- Clinicians can specify any or all of the following:
- Total vitamin B12
- Holotranscobalamin
- Methylmalonic acid
- Homocysteine
⚠️ Who Qualifies for 66842 (Clinical Need Criteria)
- Patients with:
- Inconclusive initial results
- Low dietary intake (e.g. vegans, elderly)
- Family history of B12 deficiency
- Prior abdominal/pelvic radiotherapy
- Gastrointestinal surgery (e.g. bariatric, ileal resection)
- Recreational nitrous oxide use
- Medical conditions (e.g. pernicious anaemia, IBD, pancreatic insufficiency)
Management
- Concurrent folate or iron deficiency blunts the response—replace concurrently if low.
- Never give folic acid alone when B12 deficiency is possible; it can mask haematological signs while neurological damage progresses.
- Improve diet
- B12 Fortified foods (Cereals), Animal Source – Liver, muscle meat(beef), fish, Eggs, Dairy
- Replace Vitamin B12 before replacing Folate (otherwise risk of spinal cord degeneration)
- Educate patients on:
- Medication interactions (metformin, PPIs)
- Signs/symptoms to report (fatigue, neuropathy)
- Oral
- Indication:
- Dietary B12 deficiency
- intact absorption
- but IM is mandatory when intrinsic-factor–dependent absorption is lost.
- Dose:
- Cyanocobalamin 50–150 mcg PO daily, in 1–3 divided doses.
- Follow-up:
- Recheck serum B12 and FBC in 2–3 months.
- Indication:
- IM
- Initial: 1000 mcg IM daily for 2 weeks
- 1 mg saturates transcobalamin-II binding sites
- giving more does not increase tissue uptake.
- Larger amounts are rapidly renally excreted, so the extra product adds cost without additional benefit.
- Initial: 1000 mcg IM daily for 2 weeks
| Clinical Scenario | Regimen |
|---|---|
| Pernicious anaemia or macrocytic anaemia with neurological involvement | Hydroxocobalamin 1000 mcg IM on alternate days until no further improvement → then 1000 mcg IM every 2 months lifelong |
| Pernicious anaemia or macrocytic anaemia without neurological signs | 1000 mcg IM 3×/week for 2 weeks, then 1000 mcg IM every 2–3 months lifelong |
| Confirmed B12 deficiency due to malabsorption | 1000 mcg IM 1–3×/week for 2 weeks, then 1000 mcg IM every 3 months lifelong |
| If neurological signs present (e.g., optic atrophy, peripheral neuropathy) | 1000 mcg IM on alternate days until clinical plateau, then 1000 mcg IM every 2 months |
| Asymptomatic B12 Deficiency (Low or borderline serum B12 without clinical features – no anaemia – no macrocytosis – no neurological symptoms) | Repeat serum B12 after 2 months to confirm persistent deficiency. Trial of Oral Vitamin B12 If levels and blood counts improve → continue oral therapy as needed IM if – Failure of oral therapy (no rise in B12 or improvement in MCV/anaemia) Strong risk factors: → Pernicious anaemia (positive anti-IF/anti-parietal cell antibodies) → Bariatric surgery → Inflammatory bowel disease (ileal resection/Crohn’s) → Chronic PPI or metformin use |
Expect improvement in symptoms and lab markers within 3 months of starting B12 supplementation
Lab markers (Homocysteine, methylmalonic acid, Reticulocyte Count) improve in the first week
Neurologic symptoms improve within 6 weeks to 3 months
Anemia, Leukopenia, MCV and Thrombocytopenia improve within 2 months
More than 50% of patients with B12 Deficiency related symptoms will have incomplete resolution despite treatment