How is it used?
Vitamin B12 and folate can be ordered to investigate the cause of macrocytic anaemia. They can be requested as follow-up tests when large red cells and a decreased haemoglobin concentration are found during a full blood count test. Testing for vitamin B12 and folate may be used to help evaluate the nutritional status of a patient with signs of significant malnutrition or malabsorption. This may include those with alcoholism and those with disorders associated with malabsorption such as coeliac disease, Crohn's disease and cystic fibrosis. Vitamin B12 and folate testing may also be ordered to help diagnose the cause of mental or behavioural changes, especially in the elderly.
In patients with known vitamin B12 and folate deficiencies, these tests may be used occasionally to help monitor the effectiveness of treatment. This is especially true in patients who cannot absorb vitamin B12 and/or folate and must have lifelong treatment. Either a serum or RBC folate test may be used to help detect a deficiency. Some doctors feel that the RBC folate test is more clinically relevant than serum folate but there is not widespread agreement on this. In general the laboratory where the test is ordered will provide the doctor with expert advice about which test is best to assist with obtaining accurate vitamin B12 and folate levels.
When is it requested?
Vitamin B12 and folate are usually measured when a full blood count and/or blood film, done routinely or as part of an evaluation of anaemia, indicates decreased haemoglobin concentration and/or the presence of large red cells. A high mean corpuscular volume (MCV) indicates that the red blood cells are enlarged.
Sometimes the test is ordered as part of a pre- pregnancy health screen along with a range of other tests that may help the woman ensure she is in the best health for a pregnancy.
When a person, especially an elderly person, exhibits mental or behavioural changes such as irritability, confusion, depression and/or paranoia, vitamin B12 and folate may be done to help diagnose the underlying cause. This is often part of what is called a delirium screen, it happens mostly when elderly patients are admitted to hospital. They may also be used when a patient has physical symptoms that suggest a B12 or folate deficiency, including dizziness, weakness, fatigue, or a sore mouth or tongue. When a patient has symptoms suggesting nerve damage or impairment, such as tingling, burning, or numbness in their hands or feet, a vitamin B12 test may be requested to look for vitamin B12 deficiency.
When a patient has symptoms suggesting nerve damage or impairment, such as, tingling, burning, or numbness in their hands, arms, legs, and or/feet, a vitamin B12 test may be requested to help diagnose the cause and detect the presence of a vitamin B12 deficiency.
Vitamin B12 and folate levels may sometimes be requested when a patient shows signs of malnutrition or malabsorption or is known to have a disorder that affects nutrient absorption. When a breastfed infant has a vitamin B12 or folate deficiency, then the mother may also be tested to see if she has a deficiency that is affecting both her and her child. The vitamin B12 and folate tests may be ordered for individuals being treated for malnutrition or a B12 or folate deficiency to evaluate the effectiveness of treatment.
What does the test result mean?
The doctor is looking for vitamin B12 and/or folate deficiencies. If a symptomatic patient has decreased concentrations of vitamin B12 and/or folate, then it is likely that they have some degree of deficiency. The test results will indicate the presence of the deficiency, but they do not necessarily reflect the severity of the anaemia or neuropathy associated with the deficiency or its underlying cause.
It is possible to have low B12 or folate levels on blood tests without any clinical problems. In the case of B12 measurements, this partly reflects a weakness in the standard B12 blood test (also called serum cobalamin) which does not directly measure whether there is an actual deficiency of active vitamin B12 in the cells of the body. Second-line tests that might help determine true deficiency include plasma/urine methylmalonic acid and plasma homocysteine. Serum holotranscobalamin (also known as ‘active B12’) has the potential as an alternative first-line test, but the current availability of these tests may be limited.
There are a variety of causes of vitamin B12 and/or folate deficiencies. They include:
The human body stores several years worth of vitamin B12 in the liver and it is readily available in the food supply, so a dietary deficiency of this vitamin should be rare in Australia. It may be seen sometimes with general malnutrition, and in vegans - those who do not consume any animal products including milk and eggs. It may also be seen in children of vegans and breast-fed infants. Since they do not have the stores that adults have, deficiencies in children and infants tend to occur earlier.
With the advent of fortified cereals, breads, and grain products, folate deficiency is genrally rare in Australia. Since folate is stored in tissue in smaller quantities than vitamin B12, folate must be consumed more regularly.
Both vitamin B12 and folate deficiencies may be seen with conditions that interfere with their absorption in the small intestine. These may include:
- Coeliac disease (an intolerance to gluten present in wheat and other cereal that causes inflammation and malabsorption)
- Bacterial overgrowth in the stomach and intestines
- Reduced stomach acid production (stomach acid is necessary to separate vitamin B12 from the protein in food)
- Pernicious anaemia, is occasionally the cause of vitamin B12 deficiency. Normally a molecule called intrinsic factor is made by parietal cells that line the stomach. Vitamin B12 binds to intrinsic factor in the stomach, then the resulting compound is absorbed in the intestines. With pernicious anaemia, the body's immune system destroys the intrinsic factor that is produced significantly reducing the amount of vitamin B12 that can be absorbed.
- Surgery that removes part of the stomach (and the parietal cells) or the intestines may greatly decrease absorption
Increased loss - this may be seen with:
- Heavy drinking or chronic alcoholism.
- Use of drugs such as anti-seizure medications (e.g. phenytoin), metformin, omeprazole, methotrexate.
All pregnant women need increased amounts of folate for proper fetal development. If a woman has a folate deficiency prior to pregnancy, it will be intensified during gestation, and may lead to premature birth and neural tube birth defects in the child. As part of a pre-pregnancy health screen women may also have a folate test requested. Australian guidelines recommend to women of reproductive age to consider taking folic acid supplements to reduce the risk of neural tube defects.
If a patient with a vitamin B12 or folate deficiency is being treated with supplements (or with vitamin B12 injections), then normal or elevated results indicate a response to treatment.
Is there anything else I should know?
If a patient is deficient in both vitamin B12 and folate, but only takes folic acid supplements, the vitamin B12 deficiency may be masked. The anaemia associated with both may be resolved, but the underlying neuropathy (nerve damage) will persist.
The Schilling test was once used to confirm a diagnosis of pernicious anaemia as the cause of a vitamin B12 deficiency by demonstrating abnormal B12 absorption in the small intestine. The test has been replaced, in part, by the measurement of intrinsic factor binding antibodies and parietal cell antibodies.