Vitamin B12: Optimal Testing Recommendation

Also known as: Cobalamin

Screening for vitamin B12 deficiency should only be performed if risk factors are present, which include:

  • Gastric or small intestine resections
  • Inflammatory bowel disease
  • Use of metformin for more than four months
  • Use of proton pump inhibitors or histamine H2 blockers for more than 12 months
  • Vegans or strict vegetarians
  • Adults older than 75 years

Guidelines for Test Utilization

What does the test tell me?

Vitamin B12 (or Cobalamin) is a water-soluble vitamin involved in many cellular functions, including DNA synthesis, amino acid synthesis, and fatty acid metabolism. It plays a key role in nervous system function via myelin synthesis and red blood cell maturation. Vitamin B12 was initially discovered as a product of its clinical association with pernicious anemia.

In addition to measuring vitamin B12 directly, other laboratory tests can be useful in helping to detect vitamin B12 deficiency. Homocysteine and methylmalonic acid are both elevated in the presence of vitamin B12 deficiency. [back to top]

When should I order this test?

Vitamin B12 should be ordered:

  • In patients with an abnormal CBC with a blood smear showing macrocytosis (MCV >99fL) or hypersegmented neutrophils
  • In patients with symptoms of anemia and/or of neuropathy
  • In patients being treated for vitamin B12 deficiency
  • In patients with presence of other risk factors listed in "Recommendations" [back to top]

When should I NOT order this test?

Do not order a vitamin B12 test for routine screening purposes. Vitamin B12 should be ordered only when a vitamin B12 deficiency is suspected, as described in the section "When should I order this test?". [back to top]

How should I interpret the result?

Vitamin B12 measurements are not standardized between labs. Clinicians should review the reference interval associated with each patient result. A result below the reference interval is suggestive of vitamin B12 deficiency. When vitamin B12 is normal, but clinical suspicion is high, a clinician may consider ordering additional tests for vitamin B12 deficiency, including methylmalonic acid or homocysteine, as a patient may be replete in vitamin B12 but functionally deficient. [back to top]

Is the test result diagnostic/confirmatory of the condition? If not, is there a diagnostic/confirmatory test?

A low vitamin B12 value is diagnostic of vitamin B12 deficiency. When vitamin B12 is normal, a clinician may consider ordering methylmalonic acid, which is elevated even early in vitamin B12 deficiency as well as in more advanced deficiency. Homocysteine is also often ordered as a marker of vitamin B12 deficiency as increased levels of homocysteine are a product of vitamin B12 deficiency. [back to top]

Are there factors that can affect the lab result?

Intrinsic factor antibodies can interfere with the vitamin B12 test, producing falsely elevated results. If a person has these antibodies in their blood, the results of their B12 test must be interpreted with caution. [back to top]

Are there considerations for special populations?

Older adults and vegetarians/vegans are at higher risk for vitamin B12 deficiency, as well as infants breastfed by a mother deficient in vitamin B12. [back to top]

What other test(s) might be indicated?

Two additional tests, homocysteine and methylmalonic acid, are often used to diagnose vitamin B12 deficiency instead of measuring vitamin B12 directly. These biomarkers are both elevated in the absence of sufficient vitamin B12 and frequently will reveal vitamin B12 deficiency earlier than when only measuring vitamin B12 directly.

Folate deficiency may also cause a clinical picture similar to vitamin B12 deficiency as well as causing an elevation in homocysteine, but not methylmalonic acid. [back to top]

References

Vitamin B12 Deficiency: Recognition and Management. American Family Physician. 2017 Sep 15; 96(6):384-389.

Vitamin B12 deficiency. Nature Reviews Disease Primers 3, 17040; doi 10.1038/nrdp.2017.40.

Vitamin B12 deficiency. British Medical Journal. 2014; 349:g5226; doi 10.1136/bmj.g5226.

Vitamin B12 among Vegetarians: Status, Assessment and Supplementation. Nutrients. 2016 Dec; 8(12); 767; doi 10.3390/nu8120767.


Last reviewed: March 2023. The content for Optimal Testing: the Association for Diagnostics & Laboratory Medicine’s (ADLM) Guide to Lab Test Utilization has been developed and approved by the Academy of Diagnostics & Laboratory Medicine and ADLM’s Science and Practice Core Committee.

As the fields of laboratory medicine and diagnostic testing continue to grow at an incredible rate, the knowledge and expertise of clinical laboratory professionals is essential to ensure that patients receive the highest quality and most useful laboratory tests. ADLM’s Academy and Science and Practice Core Committee have developed a test utilization resource focusing on commonly misused tests in hospitals and clinics. Improper test utilization can result in poor patient outcomes and waste in the healthcare system. This important resource geared toward medical professionals recommends better tests and diagnostic practices. Always consult your laboratory director to make sure these recommendations are appropriate for your patient population.