Vitamins, minerals and amino acids
Vitamine B12
The total amount of vitamin B12 in the blood; the first test when a B12 deficiency is suspected, with a wide gray zone.
Everything about this testWhat it measures
Vitamin B12 (cobalamin) is needed for making red blood cells, for the nervous system and for cell division. The body only gets it from animal products and fortified foods and absorbs it in the small intestine with the help of intrinsic factor, a protein from the stomach. The test measures the total B12 in the blood: the part bound to transcobalamin (the active B12 that cells can take up) and the larger part bound to haptocorrin, which cells cannot use. As a result, the value does not say exactly how much usable B12 there is. The store in the liver is large and a deficiency only develops after years.
- Vitamin B12 deficiency due to too little intake (vegan or vegetarian diet) or poor absorption (gastritis, pernicious anemia, stomach or bowel surgery, Crohn's disease)
- Anemia with red blood cells that are too large (macrocytic anemia)
- Nerve symptoms due to a B12 deficiency, such as tingling and a numb feeling
High
A raised vitamin B12 is usually due to supplements or injections and is then harmless; the body passes an excess in the urine. Without supplementation, a raised value is unusual and may go with a liver disease, a kidney disease or, rarely, a blood disease, because B12 is then released from cells or the transport proteins increase. A high value causes no symptoms on its own.
Low
A low vitamin B12 (below about 150 pmol/L) fits a deficiency. The most common causes are reduced absorption due to inflammation of the stomach lining (atrophic gastritis, in pernicious anemia due to antibodies against intrinsic factor), stomach or bowel surgery, Crohn's disease, long-term use of acid-reducing medicines or metformin, a vegan diet without a supplement, and excessive alcohol or nitrous oxide (laughing gas) use. A low-normal value, roughly between 150 and 250 pmol/L, is a gray zone: there may be a deficiency, but not necessarily. Symptoms that go with a deficiency are tiredness, shortness of breath, tingling or a numb feeling in the hands and feet, balance problems, memory complaints and a sore tongue.
Preparation
- Do you take biotin (vitamin B8, often in hair and nail supplements)? Stop at least 2 days before the sample is taken, or 4 days for a high dose.
If your result is abnormal
With a clearly low value, the GP usually starts supplementation with tablets or injections and looks for the cause. Sometimes the GP also has antibodies against intrinsic factor measured, but the Dutch GP guideline (NHG) on anemia does not recommend this as standard, because it usually does not change the approach. With a low-normal value and a persistent suspicion, the GP has methylmalonic acid (or sometimes active B12) measured before drawing a conclusion; with severe nerve symptoms or an unclear cause, a referral to the internist (specialist in internal medicine) or neurologist follows.
Reliability
A clearly low value reliably points to a deficiency and a comfortably normal value makes a deficiency unlikely, but the gray zone in between is wide. According to the Dutch GP guideline (NHG) on anemia, with a low-normal or slightly low value and a persistent suspicion, the GP has methylmalonic acid measured: it builds up when the cells have too little active B12, and a normal result virtually rules out a deficiency. Some laboratories instead measure active B12 (holotranscobalamin), the part that cells can take up. The total B12 value is not suitable for monitoring treatment with injections; the value is then always high.
Limitations
- Supplements, injections and fortified foods raise the value; a multivitamin can also mask a deficiency.
- Biotin in high doses (hair and nail supplements) can interfere with the measurement; stop biotin at least two days before the blood draw.
- Pregnancy, the contraceptive pill and a folic acid deficiency lower the value without a real B12 deficiency; a liver disease or blood disease raises the value.
- Reference values and units (pmol/L or ng/L) differ between laboratories; depending on the laboratory, the gray zone lies between about 150 and 250 pmol/L.
How common is this
In general practice, anemia due to a deficiency of vitamin B12 or folic acid newly occurs in about 2 in 1000 people per year, mainly in older people, in people with an autoimmune inflammation of the stomach and with a one-sided diet. A B12 deficiency without anemia is more common, especially at older age and with long-term use of acid-reducing medicines or metformin; exactly how common depends on the threshold value used.
Who is this useful for
Useful with symptoms that fit a B12 deficiency (tiredness, tingling or a numb feeling in the hands or feet, balance or memory complaints, a sore tongue), with anemia with red blood cells that are too large, with a vegan or vegetarian diet without a supplement, after stomach or bowel surgery, with Crohn's disease or celiac disease, with long-term use of acid-reducing medicines or metformin, and with nitrous oxide use. In adults without symptoms and with a varied diet, routine measurement is not needed, and in people who already take or inject B12, the value says little.
Downsides and risks
- A low-normal value does not tell the whole story: without methylmalonic acid or active B12, a deficiency can be missed or, on the contrary, wrongly assumed.
- Starting B12 on your own initiative before the cause is known makes further testing for the cause (for example pernicious anemia) more difficult.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results