Version July 2025
ALGORITHM —
INITIAL EVALUATION —
Serum vitamin B12 (cobalamin) is usually measured in conjunction with serum folate to determine the etiology of macrocytosis (mean corpuscular volume [MCV] >100 fL), macrocytic anemia, or mild pancytopenia. Serum vitamin B12 and folate levels may also be obtained as part of the evaluation of overall nutritional status, particularly in patients with underlying gastrointestinal disorders or other conditions associated with deficiencies of these vitamins (table 1 and table 2). Vitamin B12 alone may be measured to evaluate peripheral neuropathy, altered mental status, or other unexplained neurologic abnormalities. (See "Clinical manifestations and diagnosis of vitamin B12 and folate deficiency" and "Causes and pathophysiology of vitamin B12 and folate deficiencies".)
Low vitamin B12
Vitamin B12 level <200 pg/mL — A serum vitamin B12 level <200 pg/mL (<148 pmol/L) is consistent with vitamin B12 deficiency. The goal of the evaluation is to determine the cause (table 2). (See 'Determine the etiology' below.)
Vitamin B12 level 200 to 300 pg/mL — Serum vitamin B12 levels ranging from 200 to 300 pg/mL (148 pmol/L to 221 pmol/L) are in the borderline range and typically require additional testing. (See "Clinical manifestations and diagnosis of vitamin B12 and folate deficiency", section on 'Additional testing for selected individuals'.)
Measure:
●Serum methylmalonic acid (MMA)
●Serum homocysteine
MMA and homocysteine are metabolic intermediates that accumulate when a step in the recycling of folate or homocysteine/methionine is blocked (as occurs with vitamin B12 deficiency). For MMA and homocysteine reference ranges, laboratory-specific, assay-specific cutoffs should be used.
●MMA and homocysteine elevated – vitamin B12 deficiency confirmed (does not eliminate the possibility of folate deficiency)
●MMA and homocysteine normal – vitamin B12 and folate deficiencies excluded
●MMA normal and homocysteine elevated – vitamin B12 deficiency excluded; consistent with folate deficiency (see 'Low folate' below)
There may be substantial fluctuations in the measured MMA and homocysteine levels, and unexpectedly normal or abnormal levels should be repeated. MMA is excreted in urine and may be elevated with impaired kidney function despite normal vitamin B12 levels. If vitamin B12 (and folate) deficiency is excluded by metabolite testing, evaluate for other causes of macrocytosis, anemia, and/or hypersegmented neutrophils, if present.
Determine the etiology — If a diagnosis of vitamin B12 deficiency is confirmed, the cause(s) for the deficiency should be determined unless the patient has a previously diagnosed condition associated with vitamin B12 deficiency (table 2). The specific tests required (and their sequence) depend on the clinical setting, the abnormalities present on the complete blood count (CBC), and the results of initial testing.
●Review medications for agents that block or impair vitamin B12 absorption (table 2).
●Assess for a gastrointestinal or dietary condition associated with vitamin B12 deficiency, including:
•Reduced dietary intake (eg, strict vegan diet without vitamin B12 supplementation)
•Bariatric, gastric, or intestinal surgery (see "Bariatric surgery: Postoperative nutritional management", section on 'Micronutrient deficiency, supplementation, and repletion')
•Inflammatory bowel disease or other causes of malabsorption (see "Endoscopic diagnosis of inflammatory bowel disease in adults" and "Small intestinal bacterial overgrowth: Clinical manifestations and diagnosis")
•Celiac disease, particularly if patient is nonadherent to a gluten-free diet (see "Diagnosis of celiac disease in adults")
•Pancreatic insufficiency (see "Exocrine pancreatic insufficiency")
●For patients with unexplained B12 deficiency, measure antibodies to intrinsic factor to assess for pernicious anemia. (See "Clinical manifestations and diagnosis of vitamin B12 and folate deficiency", section on 'Autoantibodies to intrinsic factor'.)
Additional evaluation depends on the patient's clinical status and symptoms. (See "Clinical manifestations and diagnosis of vitamin B12 and folate deficiency", section on 'Determining the underlying cause of vitamin B12 deficiency' and "Treatment of vitamin B12 and folate deficiencies".)
Low folate
Folate level <2 ng/mL — In the absence of recent anorexia or prolonged fasting, a serum folate level <2 ng/mL (<4.5 nmol/L) is consistent with folate deficiency. (See 'Determine the etiology' below.)
In patients with concomitant vitamin B12 deficiency, vitamin B12 should also be administered. Folic acid cannot correct the neurologic deficits caused by vitamin B12 deficiency; these deficits may become progressive and irreversible if not treated with supplemental vitamin B12.
Folate level 2 to 4 ng/mL — A serum folate level of 2 to 4 ng/mL (4.5 to 9.1 nmol/L) does not definitively diagnose folate deficiency. Further testing is required to confirm or exclude the diagnosis of folate deficiency and to exclude vitamin B12 deficiency.
Measure:
●Serum MMA
●Serum homocysteine
MMA and homocysteine are metabolic intermediates that accumulate when a step in the recycling of folate or homocysteine/methionine is blocked (as occurs with vitamin B12 deficiency). For MMA and homocysteine reference ranges, laboratory-specific, assay-specific cutoffs should be used.
●MMA and homocysteine elevated – vitamin B12 deficiency confirmed; does not exclude folate deficiency
●MMA and homocysteine normal – vitamin B12 and folate deficiencies excluded
●MMA normal and homocysteine elevated – vitamin B12 deficiency excluded; consistent with folate deficiency
There may be substantial fluctuations in the measured MMA and homocysteine levels, and unexpectedly normal or abnormal levels should be repeated. If folate (and vitamin B12) deficiency is excluded by metabolite testing, evaluate for other causes of macrocytosis, anemia, and/or hypersegmented neutrophils, if present.
Determine the etiology — If a diagnosis of folate deficiency is made, additional evaluation for one of the underlying conditions (table 1) depends on the patient's clinical status and symptoms. Assess dietary intake, medications, and possibility of malabsorption associated with folate deficiency (eg, celiac disease, inflammatory bowel disease).
REFERENCE RANGE —
The normal reference range for vitamin B12 is approximately >300 pg/mL (>221 pmol/L) and for serum folate >4 ng/mL (>9.1 nmol/L) but can vary depending on the patient population and clinical laboratory. Interpretation of a specific abnormal test result should be based upon the reference range reported by the laboratory. A normal serum folate concentration excludes folate deficiency, as long as the patient has not had a recent folate-replete meal.
CITATIONS —
The supporting references for this content are accessible in the linked topics.
