Vitamin B12 Deficiency Diagnosis and Management

Diagnostic Criteria

The American College of Physicians recommends measuring serum vitamin B12 level immediately, with deficiency defined as <150 pmol/L or <203 ng/L, and if borderline, obtaining methylmalonic acid level (>271 nmol/L confirms deficiency) 1
The American College of Physicians suggests checking serum folate and RBC folate levels, with deficiency indicated by serum folate <10 nmol/L or RBC folate <305 nmol/L 1
Obtaining reticulocyte count is recommended to differentiate megaloblastic from non-megaloblastic causes, with a normal/low count suggesting megaloblastic anemia 1
Measuring TSH and free T4 is advised to exclude hypothyroidism as a cause of macrocytosis 1
Checking CRP and creatinine is suggested to assess for inflammatory conditions or renal failure 1
Evaluating for other dysplastic features that might suggest myelodysplastic syndrome is recommended, though this is less likely given the clinical presentation 3

The American College of Physicians recommends never initiating folate supplementation before ruling out and treating vitamin B12 deficiency, as this can precipitate subacute combined degeneration of the spinal cord—an irreversible neurological complication 1, 2
For confirmed B12 deficiency without neurological symptoms, administering cyanocobalamin 1 mg (100 mcg) intramuscularly three times weekly for 2 weeks, followed by 1 mg every 2-3 months for life, is recommended 1
For patients with neurological symptoms, administering hydroxocobalamin 1 mg intramuscularly on alternate days until no further improvement, then continuing with 1 mg every 2 months, is advised 1
The European Crohn’s and Colitis Organisation suggests that only after B12 deficiency is excluded or treatment initiated, oral folic acid 5 mg daily for a minimum of 4 months should be given 4, 1

Repeating complete blood counts is recommended to monitor response to treatment 1, 2
An acceptable response is defined as a hemoglobin increase of at least 2 g/dL within 4 weeks 1, 2
The American College of Physicians suggests reviewing current medications for azathioprine, methotrexate, or hydroxyurea, which can cause macrocytosis 1, 2
In inflammatory conditions, ferritin may be falsely elevated despite true iron deficiency, and checking transferrin saturation and RDW is recommended 1, 2