Approach

Neurologic disease associated with vitamin B12 deficiency may be irreversible; therefore, early detection is critical in preventing permanent neurologic damage.

Serum vitamin B12 is a standard initial diagnostic test.[1]​ Low serum vitamin B12 (generally <200 picograms/mL) in the presence of specific clinical symptoms provides evidence of deficiency. Supplementary tests, including markers of tissue deficiency (homocysteine, methylmalonic acid [MMA], and holotranscobalamin), improve earlier diagnosis, and may be considered.

Optimal use of serum vitamin B12 and tissue markers is undefined, but general guidance will help the clinician to determine whether a patient has true vitamin B12 deficiency.

At-risk groups

The clinician should take certain risk factors into account when considering who should be tested for vitamin B12 deficiency.

Advancing age increases risk.[11][12][13][14][15][16]​ Older patients who present with clinical features of deficiency may have vitamin B12 levels within the reference range; further testing may be warranted.[1][64]

Chronic use of certain medications (specifically metformin, H2 receptor antagonists, proton-pump inhibitors, and anticonvulsants) can also place a patient at risk of deficiency.[31][32][37]​​[38][49][51][53]

Recreational nitrous oxide (N₂O) misuse may increase the risk of vitamin B12 deficiency.[39][40]

Patients with chronic gastrointestinal (GI) illnesses that can cause malabsorption or inadequate absorption, including Crohn disease and celiac disease, and those with a history of upper GI surgery, including gastrectomy, gastric bypass, or ileectomy, should be tested for vitamin B12 deficiency.[27][45][46][47]

Vegans or strict vegetarians who do not take additional dietary supplementation are at risk of developing vitamin B12 deficiency.[26][48]

Vitamin B12 deficiency may be seen in 20% to 30% of women during pregnancy, and is particularly common if the woman is vegetarian or vegan.[11][29]

Symptoms and signs

Patients with unexplained neurologic disease (specifically, decreased vibration sense, gait abnormalities, and peripheral neuropathies) should be tested for vitamin B12 deficiency. Neuropsychiatric complaints such as depression and dementia may alert the clinician to an occult vitamin B12 deficiency.[11]

Late signs of vitamin B12 deficiency include angular cheilitis, glossitis of the tongue, and signs of frank anemia and thrombocytopenia.

Initial diagnostic testing

Serum vitamin B12 remains a useful initial diagnostic test due to its widespread availability and familiarity. However, caution must be used when interpreting the values, as there are no well-defined cut-offs for deficiency.[65]

The likelihood of vitamin B12 deficiency can be defined according to the serum vitamin B12 level as follows:[1][2]

  • Probable vitamin B12 deficiency: <200 picograms/mL

  • Possible vitamin B12 deficiency: 201 to 350 picograms/mL

  • Unlikely vitamin B12 deficiency: >350 picograms/mL.

Vitamin B12 level >350 picograms/mL does not exclude vitamin B12 deficiency, particularly if pernicious anemia is suspected. Spuriously normal or high serum vitamin B12 levels have been reported in patients with pernicious anemia due to anti-intrinsic factor antibody (anti-IFAB) interference in laboratory assays.[3][4][5][6]

A complete blood count with peripheral smear is useful to determine whether there is evidence of macrocytosis and frank anemia, leukopenia, or thrombocytopenia. This suggests a more severe and prolonged vitamin B12 deficiency. However, normal mean corpuscular volume (MCV), hemoglobin, and hematocrit are not useful to rule out vitamin B12 deficiency, as many patients with vitamin B12 deficiency may have normal hematologic parameters.

A peripheral smear may show the classic hypersegmented polymorphonucleated cells and megalocytes found in severe vitamin B12 deficiency with associated macrocytic anemia, but is not sensitive to early vitamin B12 deficiency.

In the era of folic acid fortification, concomitant folate deficiency is rare. In parts of the world where nutritional deficiencies are common, testing for concomitant folic acid deficiency and treatment can help clarify whether true vitamin B12 deficiency coexists.

A reticulocyte count may also be considered to differentiate B12 deficiency from hemolytic anemia. Vitamin B12 deficiency is associated with a low reticulocyte index, whereas hemolytic anemia is associated with a high reticulocyte index.

Clinical assessment of deficiency severity

The severity of the deficiency can be graded clinically as follows:

  • Mild to moderate hematologic manifestations: usually asymptomatic with normal hematocrit and an MCV that is at the upper limit of the normal range or mildly elevated.

  • Severe neurologic manifestations: subacute combined spinal degeneration, dementia, or cognitive impairment. Subacute combined spinal degeneration is progressive neurologic degeneration of the posterior and lateral columns of the spinal cord; patients present with ataxia, decreased vibration sense, muscle weakness, and hyperreflexia.

  • Severe hematological manifestations: pancytopenia and marked symptomatic anemia.

Confirmatory diagnostic testing: serum vitamin B12 <200 picograms/mL

Confirmatory diagnosis is generally unnecessary and empiric treatment should begin. Clinical and serologic response in follow-up confirms vitamin B12 deficiency.[1][2]

Confirmatory diagnostic testing: serum vitamin B12 201 to 350 picograms/mL

Methylmalonic acid (MMA) can be very sensitive for vitamin B12 deficiency but falsely high levels can occur in renal disease.[1] Additionally, an abnormal MMA level is undefined. Patients with serum vitamin B12 201 to 350 picograms/mL together with an elevated MMA can be considered to have probable vitamin B12 deficiency. Diagnosis is confirmed if vitamin levels normalize and serum vitamin B12 rises with adequate vitamin B12 treatment.

MMA

Can be elevated (i.e., >0.4 micromol/L) with folate deficiency. Elevated MMA may be spurious and requires subsequent follow-up to determine whether MMA normalizes with adequate treatment. Results should be interpreted with caution in those with renal disease as this can elevate MMA levels.[1]

Homocysteine

Can be elevated (i.e., >15 micromol/L [>2.03 mg/dL]) with folate deficiency, hypothyroidism, and vitamin B12 deficiency.[1] Patients with elevated homocysteine related to vitamin B12 deficiency (when folate deficiency and hypothyroidism are ruled out) should have normalization with empiric vitamin B12 treatment.

Holotranscobalamin (hTC)

This is transcobalamin bound to vitamin B12, and can be a measure of the true functional serum vitamin B12 levels. Several studies have reported greater diagnostic accuracy with the hTC assay than with other assays measuring markers of vitamin B12 deficiency.[66] hTC may be the first marker to be detected with vitamin B12 deficiency. Levels of hTC <35 picograms/L can be consistent with vitamin B12 deficiency.[1][67][68][69]

Confirmatory diagnostic testing: serum vitamin B12 >350 picograms/mL

Patients with pernicious anemia may have spuriously normal or high serum vitamin B12 levels.[3][4][5][6]

If pernicious anemia is suspected in patients with normal or high serum vitamin B12 levels (>350 picograms/mL), further testing for MMA, homocysteine, and hTC should be carried out to determine if vitamin B12 deficiency is present.[1]

Determining the underlying cause of vitamin B12 deficiency

Once the diagnosis of vitamin B12 deficiency is established, an etiology should be sought. While treatment remains the same, vitamin B12 deficiency can lead the astute clinician to discover an underlying malabsorption process such as celiac disease or Crohn disease.

Pernicious anemia can be determined by testing for the following:

  • Anti-IFAB: only 50% sensitive, but highly specific for pernicious anemia.[2] Testing for anti-IFAB should be done before initiating vitamin B12 replacement therapy because high vitamin B12 levels may lead to false positive results.[70][71]

  • Antiparietal cell (APC) antibody: highly sensitive (85%), but has low specificity for pernicious anemia because APC antibodies may be elevated in atrophic gastritis.[2]

  • Fasting serum gastrin levels rise in gastric achlorhydria and can signify pernicious anemia.[72]

Venepuncture and phlebotomy: animated demonstration
Venepuncture and phlebotomy: animated demonstration

How to take a venous blood sample from the antecubital fossa using a vacuum needle.

Use of this content is subject to our disclaimer