Vitamin B12 deficiency: testing and treatment

SUMMARY

Vitamin B12 testing is recommended for individuals with clinical signs and symptoms suggestive of B12 deficiency, and when there is reasonable clinical suspicion of deficiency due to risk factors (e.g. inadequate dietary intake, malabsorptive conditions).

Where vitamin B12 testing is indicated, total serum B12 is typically the first-line test. Active B12 may be requested if total B12 results are indeterminate, or during pregnancy. If total or active B12 tests are inconclusive, methylmalonic acid or homocysteine testing may be considered; however, their concentrations may be elevated in other conditions.

In individuals with confirmed B12 deficiency, B12 supplementation is required, with the choice of formulation, duration and dosage guided by the underlying cause and severity of the deficiency and patient preference.

 

Introduction

Vitamin B12 deficiency has been recognised as a health concern for over a century. In Australians aged over 50 years, the prevalence of vitamin B12 deficiency ranges from 5.2 to 6.3%,^1,2 and refugees from Sudan, Bhutan, Iran, Iraq and Afghanistan are at particularly high risk (20 to 30%).³

Vitamin B12 is a water-soluble vitamin that plays an essential role in cellular metabolism and is crucial for neurological function, blood cell production and DNA synthesis. Vitamin B12 deficiency can lead to neurological problems and haematological abnormalities that are often reversible if diagnosed and treated early. Severe deficiency can cause subacute combined degeneration of the spinal cord, which may not be fully reversible even after the deficiency is corrected.

Diagnosing vitamin B12 deficiency remains challenging because, despite multiple available tests, none is universally reliable or diagnostic in isolation.⁴ It is essential to consider the patient's risk factors or symptoms in conjunction with various biochemical markers to establish a diagnosis. The primary biomarkers traditionally employed are total serum vitamin B12 (cobalamin) and active B12 (holotranscobalamin). Active B12 represents the metabolically active fraction of vitamin B12 bound to the transport protein transcobalamin. Methylmalonic acid and homocysteine are metabolic intermediates that accumulate in vitamin B12 deficiency and, in the appropriate clinical context, can support a diagnosis of B12 deficiency.

This article discusses the indications for B12 testing, types of laboratory tests, and treatment of B12 deficiency.

 

Causes of vitamin B12 deficiency

Common causes of vitamin B12 deficiency include autoimmune gastritis, pernicious anaemia, malabsorption due to drugs or disorders of the stomach or small intestine, and dietary insufficiency. The most important risk factors and the mechanism by which they cause B12 deficiency are listed in Table 1.

Table 1 Mechanisms and risk factors for vitamin B12 deficiency

Mechanism for B12 deficiency	Risk factors
Decreased intrinsic factor	Atrophic gastritis Helicobacter pylori–associated gastritis Pernicious anaemia secondary to late-stage autoimmune gastritis Bariatric surgery Gastrectomy
Inadequate dietary intake	Excessive alcohol consumption Older age Vegan or vegetarian diet Breastfed infants of mothers with a vegan or vegetarian diet Eating disorders
Decreased ileal absorption	Coeliac disease Crohn disease Ileum resection Radiation enteritis Small intestine bacterial overgrowth (e.g. in diabetes)
Drugs	Metformin5 Proton pump inhibitors (used for more than 2 years)5,6 Histamine H2-receptor blockers (used for more than 2 years)5 Antiepileptic drugs (pregabalin, primidone, topiramate)7 Colchicine8
Other	Recreational nitrous oxide use

 

When to test vitamin B12

Healthcare providers should assess the patient's symptoms, medical history, risk factors and clinical findings to determine the appropriateness of B12 testing.

Vitamin B12 testing should only be performed when there is reasonable clinical suspicion of deficiency due to the presence of risk factors (Table 1) or clinical findings suggestive of B12 deficiency (Table 2).

Table 2 Clinical manifestations of vitamin B12 deficiency

Category	Clinical manifestations
Nonspecific or generalised symptoms	Fatigue Weakness Stomatitis Weight loss Anorexia
Psychiatric symptoms	Irritability Personality changes Memory impairment Depression Delirium Psychosis
Haematological abnormalities	Megaloblastic anaemia (macrocytic red cells on blood film, hypersegmented neutrophils) Pancytopenia
Neurological symptoms and findings	Paraesthesia Peripheral neuropathy Subacute combined degeneration of the spinal cord (loss of vibration and proprioception in hands and feet, generalised sensory loss, cognitive and motor impairment)
Infants of mothers with B12 deficiency	Movement disorders Irritability Feeding difficulties Permanent developmental disabilities [NB1]
NB1: There is an increased risk of neural tube defects in infants of mothers with vitamin B12 deficiency.9

Vitamin B12 testing is indicated to determine the aetiology of blood count or blood film abnormalities, such as macrocytic anaemia, pancytopenia, or the presence of hypersegmented neutrophils.

In cases of unexplained neurological or psychiatric symptoms, even in the absence of anaemia, vitamin B12 testing may be appropriate as deficiency can be present without haematological abnormalities. It is reasonable to exclude B12 deficiency in older adults being investigated for cognitive decline.¹⁰

Vitamin B12 testing is not recommended for:

• asymptomatic people with no risk factors
• routine screening in the general population
• patients already taking B12 supplements (unless assessing adherence or suspected malabsorption)
• patients who received a normal B12 test result within the last 12 months, unless clinical status has changed significantly.

For people with suspected dietary B12 deficiency, a detailed dietary history should be obtained to confirm dietary inadequacy. If there is any uncertainty about the diagnosis, investigations to exclude other potential causes may be required.

A diagnosis of autoimmune gastritis should be considered in people with otherwise unexplained B12 deficiency, especially if they have upper gastrointestinal symptoms (e.g. nausea, epigastric pain, bloating), or a history of other autoimmune disorders.¹¹ If autoimmune gastritis is suspected, testing for anti-intrinsic factor antibodies should be performed. If anti-intrinsic factor antibodies are negative, further investigations, such as anti-gastric parietal cell antibodies, gastrin concentrations and referral for a gastroscopy, should be considered.

If the cause of vitamin B12 deficiency remains unknown, consider serological testing for coeliac disease and Helicobacter pylori.¹²

 

Laboratory tests for vitamin B12 deficiency

There is currently no definitive test for diagnosing vitamin B12 deficiency. Tests cannot be solely relied upon to confirm or exclude deficiency; results must be interpreted alongside clinical findings. Pathology reports, including B12 test results, may be available on a patient's My Health Record unless the person has opted out.

Total B12 (cobalamin)

The most widely available and commonly used test to assess for vitamin B12 deficiency is total serum vitamin B12 (cobalamin). This test measures both the active and inactive forms of B12 bound to its transport proteins haptocorrin (80%) and transcobalamin (20%). As the most cost-effective option, it is typically the first test performed when assessing B12 concentrations, except in pregnancy when active B12 is tested (Figure 1). A low total B12 concentration confirms the clinical suspicion of B12 deficiency (Table 3), and no further testing is required.

Table 3 Total and active B12 tests and interpretation of results¹³

Test	Result	Interpretation of result
Total B12 (cobalamin)	Less than 133 pmol/L	Deficiency likely
	133 to 258 pmol/L	Indeterminate
	Greater than 258 pmol/L	Deficiency unlikely
Active B12 (holotranscobalamin)	Less than 25 pmol/L	Deficiency likely
	25 to 70 pmol/L	Indeterminate
	Greater than 70 pmol/L	Deficiency unlikely

Low total B12 concentrations can occur without true tissue (functional) deficiency, most commonly due to low concentrations of haptocorrin. Low haptocorrin concentrations may be hereditary,¹⁴ but are more often acquired, as seen in pregnancy¹⁵ and with the use of estrogen-containing drugs. In these situations, measuring active B12 concentrations is recommended.

High total B12 concentrations masking tissue deficiency may occur in the setting of certain conditions due to increased haptocorrin (e.g. acute inflammatory conditions, myeloproliferative neoplasms, liver disease)¹⁶ or the presence of anti-intrinsic factor antibodies that can cause interference with the B12 assay, resulting in an artefactually high total B12 concentration.¹⁷

Active B12 (holotranscobalamin)

Active B12 (holotranscobalamin) testing measures the biologically active fraction of B12 available to cells and is particularly useful for assessing potential deficiency in individuals with indeterminate total serum B12 concentrations. Measuring active B12 is recommended in pregnancy due to the physiological decrease in total B12 concentration, even in the absence of true deficiency.

While active B12 testing has higher sensitivity compared with total B12, its routine use is limited by cost and availability.

Methylmalonic acid and homocysteine

Methylmalonic acid and homocysteine are functional biomarkers that help assess tissue-level B12 status when other B12 tests are indeterminate. Normal methylmalonic acid and homocysteine concentrations exclude vitamin B12 deficiency. Elevated methylmalonic acid or homocysteine may suggest functional B12 deficiency; however, concentrations can also be elevated in other conditions. Homocysteine concentrations may be elevated in folate and B6 deficiency, chronic kidney disease, hypothyroidism, or inadequate activity of the enzyme methylene tetrahydrofolate reductase due to genetic defects. Methylmalonic acid concentrations are typically normal in folate deficiency but may be elevated in chronic kidney disease or rare enzymic defects (e.g. methylmalonyl-CoA mutase deficiency).

If vitamin B12 deficiency due to recreational use of nitrous oxide is suspected, homocysteine or methylmalonic acid should be the initial test as nitrous oxide inactivates the vitamin B12 molecule, which can result in normal total and active B12 concentrations despite tissue deficiency.

Medicare Benefits Schedule rebates for B12 testing

In Australia, the Medicare Benefits Schedule (MBS) was updated in July 2025 to clarify appropriate testing pathways for vitamin B12 and to reduce clinically unnecessary testing.¹⁸ Total B12, active B12, methylmalonic acid and homocysteine tests are eligible for an MBS rebate only once every 11 months. A new MBS item can be used for patients who require more frequent testing, such as those at risk of deficiency or with initially inconclusive results.

 

Treatment of vitamin B12 deficiency

Treatment of vitamin B12 deficiency is warranted in patients with confirmed deficiency and is dependent on the underlying cause and severity. A trial of supplementation may be considered in people with indeterminate B12 concentrations (especially those with risk factors for deficiency), following careful clinical assessment, with ongoing evaluation and deprescribing where appropriate. The choice of treatment should consider the route of administration, dose, duration, cost and patient preference (Table 4).

Table 4 Treatment of vitamin B12 deficiency

Cause of B12 deficiency	Route of administration	Initial or loading regimen	Maintenance regimen	Key notes and practice points
Irreversible malabsorption: autoimmune gastritis pernicious anaemia total gastrectomy terminal ileum resection	Intramuscular	Hydroxocobalamin 1000 micrograms intramuscularly on alternate days for 1 to 2 weeks then weekly for 4 to 8 weeks	Hydroxocobalamin 1000 micrograms intramuscularly every 3 months lifelong	Oral route unreliable even at high doses; current Australian guidelines recommend intramuscular B12 lifelong19
Significant but potentially reversible malabsorption: coeliac disease Crohn disease partial gastrectomy bariatric surgery	Intramuscular preferred initially Oral may be considered	Same loading regimen as for 'Irreversible malabsorption' above	Hydroxocobalamin 1000 micrograms intramuscularly every 3 months, or oral cyanocobalamin at least 1 mg daily if proven effective	If oral supplementation used, must confirm biochemical correction (total serum B12 with or without active B12 or methylmalonic acid) before switching to maintenance regimen
Severe symptomatic deficiency: symptomatic anaemia neurological involvement subacute combined degeneration of the spinal cord	Intramuscular (mandatory)	Aggressive loading: hydroxocobalamin 1000 micrograms intramuscularly on alternate days until no further improvement (often at least 2 to 3 weeks)	Hydroxocobalamin 1000 micrograms intramuscularly every 3 months lifelong	Rapid parenteral therapy needed to prevent irreversible neurological damage; magnetic resonance imaging and symptoms often improve within days to weeks Check folate and iron concentrations as intensive loading may unmask folate deficiency, cause iron deficiency and precipitate hypokalaemia
Drug induced (e.g. long-term metformin, proton pump inhibitors, H2-receptor blockers, colchicine)	Intramuscular or high-dose oral	Oral cyanocobalamin 1 mg daily or intramuscular hydroxocobalamin as per 'Irreversible malabsorption' regimen above	Continue only until offending drug stopped and deficiency corrected	Retest to confirm correction of deficiency 3 to 6 months after stopping the offending drug; many patients can stop supplementation once deficiency is corrected
Pure dietary insufficiency (e.g. vegetarian or vegan diet, malnutrition)	High-dose oral preferred	Oral cyanocobalamin 1 mg daily (no loading needed)	Oral cyanocobalamin 1 mg daily or 1 to 2 mg weekly long term	Oral supplementation more cost effective and may be preferred by patients; intramuscular used only if poor adherence or social barriers
Recreational nitrous oxide use	Intramuscular	Various regimens suggested (e.g. hydroxocobalamin 1000 micrograms intramuscularly daily for 1 week, followed by 1000 micrograms weekly for 4 weeks, then 1000 micrograms monthly until recovered, for up to 3 months)20	Treat acutely, then review after stopping treatment	Ongoing replacement rarely needed once nitrous oxide use stopped and B12 stores repleted; neurological recovery may be slow and incomplete

Lifelong intramuscular vitamin B12 replacement is recommended for patients with deficiency caused by autoimmune gastritis, pernicious anaemia, total gastrectomy or terminal ileum resection (Table 4),¹⁹ because these conditions irreversibly impair gastrointestinal absorption, making oral B12 unreliable or ineffective even at high doses. There is accumulating evidence that sublingual or oral B12 may be as effective as parenteral B12 therapy; a 2024 meta-analysis reported that sublingual and oral B12 supplementation appear to be as effective as intramuscular injections in improving B12 status.²¹ Further high-quality randomised controlled trials are required to confirm dosing strategies and long-term outcomes.

 

Monitoring of vitamin B12 replacement

Most people who start vitamin B12 replacement notice an improvement in symptoms within 2 weeks, although they may take several months to resolve completely. Neurological symptoms improve gradually over 6 to 12 weeks, and bone marrow recovery begins within 3 to 5 days, marked by the onset of reticulocytosis. Haemoglobin concentrations usually return to the normal range within 4 to 8 weeks of starting treatment.²²

People on oral B12 replacement should be reviewed after 2 to 3 months of therapy to reassess their symptoms and ensure ongoing adherence with B12 replacement. Repeating vitamin B12, methylmalonic acid or homocysteine testing could be considered after 3 to 6 months of oral therapy to confirm correction of the B12 deficiency, particularly in people with malabsorption.

A decision to deprescribe vitamin B12 therapy should only be made if a reversible cause was identified and has been corrected. Patients should be advised to present for reassessment if symptoms return because this could indicate that the deficiency has recurred, and further treatment may be required.

 

Conclusion

Vitamin B12 testing is recommended for individuals with clinical signs and symptoms suggestive of B12 deficiency, and when there is reasonable clinical suspicion of deficiency due to risk factors such as malabsorption. Total serum B12 is commonly the first-line test, while active B12 may be requested if total B12 results are indeterminate or during pregnancy. In people with confirmed B12 deficiency, supplementation is required, with the choice of formulation, duration and dosage dependent on the underlying cause and severity of the deficiency and patient preference.

This article was finalised on 17 February 2026.

Conflicts of interest: Darya Bedz has no conflicts of interest to declare.

Cecily Forsyth has been on advisory boards for Novartis and GSK. Cecily has received fees from AstraZeneca, Janssen-Cilag, Novartis and AbbVie to speak at or attend educational meetings.

This article is peer reviewed.

 

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