The Test Sample
What is being tested?
Vitamin B12 and folate are two vitamins that are part of the B complex of vitamins. They are necessary for normal red blood cell (RBC) formation, repair of tissues and cells, and synthesis of DNA, the genetic material in cells. Both are nutrients that cannot be produced in the body and must be supplied by the diet.
Vitamin B12 and folate tests measure vitamin levels in the liquid portion of the blood (serum or plasma) to detect deficiencies. Sometimes the amount of folate inside red blood cells may also be measured.
Folate refers to a natural occurring form of the vitamin, whereas folic acid refers to the supplement added to foods and drinks. It is found in leafy green vegetables, citrus fruits, dry beans and peas, liver, and yeast. Vitamin B12, also called cobalamin, is found in foods from animals, such as red meat, fish, poultry, milk, yogurt, and eggs. In recent years, fortified cereals, breads, and other grain products have also become important dietary sources of B12 and folate (identified as "folic acid" on nutritional labels).
A deficiency in either B12 or folate can lead to macrocytic anemia, where red blood cells are larger than normal. Megaloblastic anemia, a type of macrocytic anemia, is characterized by the production of fewer but larger RBCs called macrocytes, in addition to some cellular changes in the bone marrow. Other laboratory findings associated with megaloblastic anemia include decreased white blood cell (WBC) count and platelet count.
B12 is also important for nerve health and a deficiency can lead to varying degrees of neuropathy, nerve damage that can cause tingling and numbness in the affected person's hands and feet.
Folate is necessary for cell division such as is seen in a developing fetus. Folate deficiency during early pregnancy can increase the risk of neural tube defects such as spina bifida in a growing fetus.
B12 and folate deficiencies are most often caused by not getting enough of the vitamins through the diet or supplements, inadequate absorption, or by increased need as seen in pregnancy:
- Dietary deficiencies—these are uncommon in the United States because many foods and drinks are supplemented with these vitamins, which are stored by the body. Adults typically have several years' worth of vitamin B12 stored in the liver and about 3 months of stored folate. Dietary deficiencies do not usually cause symptoms until stores of the vitamins within the body have been depleted. B12 deficiencies are sometimes seen in vegans (those who do not consume any animal products) and in their breast-fed infants.
- Inadequate absorption—vitamin B12 absorption occurs in a series of steps. B12 is normally released from food by stomach acid and then, in the small intestine, is bound to intrinsic factor (IF), a protein made by parietal cells in the stomach. This B12-IF complex is then absorbed by the small intestine, bound by carrier proteins (transcobalamins), and enters the circulation. If a disease or condition interferes with any of these steps, then B12 absorption is impaired.
- Increased need—this can be seen with a variety of diseases and conditions. Increased demand for folate occurs when a woman is pregnant or nursing, in early childhood, with cancers, or with chronic hemolytic anemias.
For more, see the article on Vitamin B12 and Folate Deficiencies.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Fasting for 6-8 hours before sample collection may be required. Certain medicines may affect the test results; your healthcare provider will advise you on which ones to stop taking. Ask your health practitioner or lab for specific instructions.