Low vitamin B12: what your result means

What the B12 test measures

Vitamin B12 is needed to make healthy red blood cells and to keep nerves working. Your body cannot make it, so it comes from food such as milk, curd, eggs, fish and meat. The blood test measures how much B12 is in your serum. Reports may show it in pg/mL or pmol/L (pg/mL multiplied by 0.74 gives pmol/L). The result is only one part of the picture. A borderline value can still matter if you have symptoms, so doctors sometimes add tests such as methylmalonic acid (MMA) or homocysteine to see whether your body is truly short of B12.

Common causes of low B12

In many people the cause is simple and easy to fix, but it is still worth finding out why the level is low, because the treatment depends on the cause. Vegetarian diets with little milk or eggs are a common reason in South Asia. Problems with absorption become more likely with age, and some people have more than one cause at the same time.

• A vegetarian or vegan diet with little dairy or egg
• Long-term use of metformin or acid-reducing medicines
• Pernicious anaemia, an autoimmune condition that blocks absorption
• Stomach or bowel surgery, coeliac disease or Crohn's disease
• Heavy alcohol use and older age

Symptoms and how B12 links to MCV and folate

Some people with low B12 feel well. Others notice tiredness, breathlessness, a sore red tongue, pins and needles in the hands or feet, poor balance, low mood or memory problems. On a blood count, low B12 can make red cells larger than normal, so the MCV may be above 100 fL. Low folate causes the same large-cell pattern, which is why both are often tested together. A normal MCV does not rule out low B12, especially if you also have low iron, which tends to make red cells smaller.

What usually happens next

Your doctor will look at your symptoms, diet, medicines and the rest of your blood count. They may repeat the test or order extra tests to find the cause, such as antibodies for pernicious anaemia. Treatment is usually B12 tablets or injections, and the choice depends on the cause and how low the level is. Your doctor may suggest which form and dose is right for you. Nerve symptoms should be treated promptly, because nerve damage from long-standing low B12 can become permanent.

Questions to ask your doctor

• What do you think is causing my low B12?
• Do I need tests for pernicious anaemia or absorption problems?
• Should I have tablets or injections, and for how long?
• Could any of my current medicines be lowering my B12?
• When should my B12 and blood count be checked again?