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What is a Vitamin B12 test?

Vitamin B12 test is a blood test that is done to detect serum vitamin B12 levels. Vitamin B12, also known as cobalamin, is a water-soluble vitamin necessary for vital neurological functions and synthesis of DNA and blood. This vitamin is absorbed in terminal ileum (a part of small intestine) in the body, and a glycoprotein intrinsic factor is necessary for the absorption Excess amounts of B12 vitamin is stored in liver Evaluation of vitamin B12 in the serum is advised most commonly in conditions, such as anaemia and muscle weakness. However, these results may not be sufficient. Vitamin B12 in the serum is bound to the protein transcobalamin (TC). Nearly, 80% of the vitamin present on TC I is inactive, while the remaining 20% is actively transported by TC II. Often, low serum cobalamin levels are due to a deficiency of TC I.

Although low serum levels of B12 indicate a deficiency of the vitamin and higher concentrations indicate normal levels, the interpretation of intermediate concentrations is unclear.

  1. Why is Vitamin B12 test performed?
  2. How do you prepare for a Vitamin B12 test?
  3. How is a Vitamin B12 test performed?
  4. Vitamin B12 test results and normal range

Most commonly, a deficiency of vitamin B12 is suspected when a CBC test shows reduced haemoglobin levels. However, there are many other conditions that suggest the need for a vitamin B12 test. Following are the symptoms for which a vitamin B12 test is carried out:

In addition, there may be some pre-existing conditions that interfere with the absorption of vitamin B12, which may result in a deficiency. Pernicious anaemia is an autoimmune condition in which antibodies attack the intrinsic factor, thereby affecting the absorption of vitamin B12. In celiac disease, Crohn’s disease or diabetes, the absorption of vitamin B12 is reduced.

Some medications, such as metformin and proton pump inhibitors, may also interfere with the absorption of this vitamin. A history of a gastric surgery can be a cause of its malabsorption and deficiency of vitamin B12.

Furthermore, excess levels of vitamin B12 are often associated with neoplasms, haematological malignancies and liver or kidney diseases. 

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A vitamin B12 test is a simple blood test. It can be done anytime, and there is no requirement to fast before the blood sample is withdrawn from the body, unless specified by the physician. One must inform the doctor about their current medication or any supplements he or she is taking. This is necessary as some drugs interfere with vitamin B12 absorption and hence may show decreased levels.

This test is performed by collecting a blood sample from the arm of the patient. An experienced technician collects the blood sample from the arm by inserting a needle into a vein. This sample is then collected in a sterile tube. Slight pain may be sometimes felt while inserting the needle. The sample is then subjected to further analytical techniques such as immunoassays, High-performance liquid chromatography or, in recent times, radioisotope techniques to get accurate results.

Similarly, a blood smear test is a microscopic test that investigates abnormalities in blood. Hypersegmented neutrophils, which occur due to deficiency of vitamin B12, can be detected by this test and thus confirm vitamin B12 deficiency. Results are obtained within 12 hours of the test.

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Normal results: It is very challenging to define normal limits of vitamin B12 in serum because the levels are affected by several factors. A gold standard reference method is not available, but to achieve harmonisation, levels of vitamin B12 up to 300 picograms (pg)/mL can be considered as normal.

Abnormal results: A serum concentration of vitamin B12 that is less than 200 pg/mL can indicate its deficiency, whereas levels between 200 pg/mL and 300 pg/mL can be considered borderline deficiency cases. It is, however, necessary to support the test results with some additional investigation. Deficiency of vitamin B12 causes an elevation in methylmalonic acid (MMA) and homocysteine levels. Similarly, the estimation of folate levels is also advised to support vitamin B12 tests. Abnormally low levels of B12 vitamin  may indicate the following health conditions:

  • Neuropathy
  • Ataxia
  • Pernicious anaemia
  • Megaloblastic anaemia
  • Muscle weakness
  • Macular degeneration in elderly patients

Similarly, elevated levels of vitamin B12, greater than 600 pg/mL, can be an early sign for haematological disorders such as leukaemia. Hence, additional tests studies are suggested.

Vitamin B12 test is highly recommended if the symptoms mentioned above are persistent. A blood test along with other supportive tests can help in the accurate diagnosis of the underlying disease.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational perspective and is in no way a substitute for medical advice from a qualified doctor.


  1. Richard Berg and Gene Shaw. Laboratory Evaluation for Vitamin B12 Deficiency: The Case for Cascade Testing. Clin Med Res. 2013 Feb; 11(1): 7–15. PMID: 23262189
  2. Fionna O Leary and Samir Samman. Vitamin B12 in health and disease. Nutrients. 2010 Mar;2(3):299-316. PMID: 22254022
  3. Ankar A, Kumar A. Vitamin B12 Deficiency (Cobalamin) [Updated 2019 Jan 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019
  4. Andres E et al. The pathophysiology of elevated vitamin B12 in clinical practice. QJM. 2013;106(6):505-515.
  5. Georgeos Siminis et al. Measuring and tracking vitamin B12: A review of current methods with a focus on optical spectroscopy. Applied Spectroscopy Reviews. 2017;52(5):439-455.
  6. Vinod Devalia et al. Guidelines for the diagnosis and treatment of cobalamin and folate disorders.. British Journal of Haematolog. 2014;166:496-513. PMID: 24942828
  7. National Institute of Health. Office of Dietary Supplements [internet]: Bethesda (MA), US. US Department of Health and Human Services Vitamin B12
  8. Johan Frederik Hakonsen Arendt et al. Elevated plasma vitamin B12 levels as a marker for cancer: A population-based cohort study. J Natl Cancer Inst. 2013;105(23):1799-1805. PMID: 24249744
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