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Vitamin B12 Test

What is Vitamin B12?

Vitamin B12 (or cobalamin) is a naturally occurring nutrient essential to good health. Vitamin B12 can also be provided in supplements.

Vitamin B12 bound to protein in food, is released in the stomach by acid and enzymes during digestion. This free-form of the vitamin then binds to a protein produced in the stomach, known as intrinsic factor, which enables its absorption in the small intestine. The amount of vitamin B12 absorbed is dependent on the amount of available intrinsic factor. The free-form can be found in supplements; high supplemental doses of Vitamin B12 will not be completely absorbed if there is insufficient intrinsic factor for absorption.

Why is Vitamin B12 important?

Vitamin B12 is essential in the formation of red blood cells that transport oxygen in the blood to all tissues in the body; a vitamin B12 deficiency leads to a reduction in the functioning of tissues and organs. Vitamin B12 is also needed to form DNA, the molecule carrying the body's genetic code.

Vitamin B12 is critical to the development and function of brain and nerve cells. Women who are pregnant need normal levels of vitamin B12 to prevent abnormal neurological development of their baby. Moreover, the vitamin helps to break down homocysteine, an amino acid produced in the body. High levels of homocysteine damage the blood vessels and encourage the formation of blood clots, which increase the risk of a heart attack, pulmonary embolism (a blood clot in the lungs), and stroke. People that have a genetically-driven high homocysteine levels can also develop heart disease. Additionally, high levels of homocysteine have been associated with a decline in cognitive function and dementia. However, taking vitamin B12 does not appear to prevent or improve memory or heart disease.

What are the symptoms of Vitamin B12 deficiency?

People with a vitamin B12 deficiency experience symptoms including tiredness, weakness, heart palpitations, breathlessness, numbness and tingling in hands and legs, muscle weakness, visual disturbance, depression, confusion, memory loss, dementia, and a red swollen tongue.

Individuals with a vitamin B12 deficiency may also have blood test results showing abnormal red blood cell counts indicating anaemia, including megablastic anaemia and pernicious anaemia, where red blood cells are fewer in number and larger in size than normal.

Women with a vitamin B12 deficiency may experience temporary infertility and, if deficient during pregnancy, their babies may show abnormal neurological development.

Who should be tested for Vitamin B12 deficiency?

A vitamin B12 test is needed in people with symptoms, who are at risk of deficiency, or who require monitoring, such as those who

  • avoid eating animal products (i.e., vegetarians and vegans) are at risk, because the vitamin is found naturally only in meat, fish and dairy products. Similarly, drinking too much alcohol can lead to a deficiency since it decreases the appetite.
  • have inadequate levels of intrinsic factor, such as in the case of pernicious anaemia, which damages the cells in the stomach lining producing intrinsic factor that is required for vitamin B12 absorption.
  • have low stomach acid concentrations, such as with atrophic gastritis (thinning of the stomach lining).
  • take medications that reduce stomach acid (e.g., proton-pump inhibitors and antacids used to treat gastroesophageal reflux disease and peptic ulcer disease) or reduce intestinal absorption (e.g., metformin that treats diabetes).
  • have a gut or other condition that affects digestion and absorption, such as inflammatory bowel disease (ulcerative colitis and Crohn's disease), celiac disease (gluten intolerance), lupus (affects all organs and systems), and Grave's disease (a thyroid disorder).
  • have surgery of the stomach (site of vitamin B12 production) or the small intestine (site of vitamin B12 absorption).

How to test for Vitamin B12 deficiency?

Alongside symptoms of deficiency and a complete blood count, blood levels of vitamin B12 are typically used in initial laboratory assessments. However, these may not be conclusive since some people do not have symptoms or have low normal blood levels. A vitamin B12 deficiency can be confirmed by blood tests measuring levels of methylmalonic acid that is produced during breakdown of amino acids, or homocysteine; levels of these compounds rise with vitamin B12 deficiency.

It is important that all vitamin B12 supplements, including multivitamins that may contain vitamin B12, are stopped for 4 weeks before a blood sample is collected. Otherwise, the test results will not reflect your true vitamin B12 levels.

A blood test for vitamin B12 deficiency can be done from a blood sample taken in your home. A self-collection pack, complete with full instructions on how to take and collect a sample of blood, is provided by The Online Clinic. Simply, several drips of blood are taken from your little finger, and collected in a collection tube. Once labelled, the blood sample is posted to the named address for laboratory analysis. Test results will be sent to your doctor at The Online Clinic who will then interpret the findings for you.

Results are usually 24 hours after the sample is received.

What do Vitamin B12 test results indicate?

The values indicating normal vitamin B12 levels and deficiency vary depending on test method and laboratory. Using blood serum or plasma, Vitamin B12 values lower than 148 or 185 pmol/L are considered to be below normal. Levels of serum methylmalonic acid greater than 0.271 micromol/L, and levels of serum homocysteine higher than 15 micromol/L, suggest vitamin B12 deficiency. Methylmalonic acid level is considered to be most sensitive for vitamin B12 level. A suggested practice is that if a person's serum vitamin B12 level is less than 111 pmol/L, then serum methylmalonic acid levels should be measured to confirm a vitamin B12 deficiency.

What is a normal daily intake of Vitamin B12?

The body's absorption of this vitamin becomes more difficult with age. Therefore, the recommended dietary allowance (known as the RDA) of vitamin B12 varies according to a person's age. For all people who are 14 years and older, the RDA is 2.4 mcg daily; the RDA is 2.6 mcg for women during pregnancy and 2.8 mcg during breastfeeding. The RDA for children is 0.4 mcg up to age 6 months, 0.5 mcg at age 7-12 months, 0.9 mcg at age 1-3 years, 1.3 mcg at age 4-8 years, and 1.8 mcg at age 9-13 years. No upper limit of tolerability has been established for the vitamin, but vitamin B12 is generally thought to be safe.

How to maintain or achieve good Vitamin B12 levels

The most beneficial means of maintaining or achieving favourable vitamin B12 levels is through eating a diet rich in fish (including shellfish), liver, red meat, poultry, eggs, dairy products and fortified foods such as breakfast cereals and nutritional yeast. Strict vegetarians and vegans need to eat sufficient vitamin B12-fortified foods.

Pernicious anaemia and vitamin B12 deficiencies resulting from medical conditions reducing B12 absorption are usually treated with one or more vitamin B12 injections, and/or vitamin B12 supplements. Diet related deficiencies may be treated with a single B12 injection before starting vitamin B12 supplements and inclusion of vitamin B12-fortified foods. Older people can take Vitamin B12 supplements if required.

Some people with one or more risk factors may be screened for vitamin B12, such as those who have inflammatory bowel disease, use medications for diabetes or to reduce stomach acid for several months, or have had stomach or intestinal surgery, strict vegetarians and vegans, and people aged 75 years and older.

Reviewed by: Dr Loraine Haslam MBBS, DRCOG, DFSRH, LoC SDI, LoC IUT, MRCGP
GMC registration number: 4524038
Date: 29 November 2022
Next review: 28 November 2024
All UK registered doctors can have their registration checked on
The Medical Register at the GMC website.
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