Test Library

To help diagnose one cause of anemia or neuropathy; to evaluate nutritional status in some people; to monitor the effectiveness of treatment for vitamin B12 or folate deficiency

When you have an abnormal CBC with a blood smear showing large red blood cells (macrocytosis) or abnormal (hypersegmented) neutrophils; when you have symptoms of anemia (weakness, tiredness, pale skin) and/or of neuropathy (tingling or itching sensations, eye twitching, memory loss, altered mental status); when you are being treated for vitamin B12 or folate deficiency
B12 and folate levels may be ordered when a CBC, done routinely or as part of an evaluation for anemia, indicates the presence of large RBCs. This increased RBC size is reflected in the RBC indices, specifically the Mean Corpuscular Volume (MCV).

Testing for B12 and folate levels may be appropriate when a person, especially an elderly person, exhibits a sudden or unexplained mental or behavioral change, such as irritability, confusion, depression, and/or paranoia. Testing may also be ordered when someone has physical symptoms that suggest a B12 or folate deficiency, including dizziness, weakness, fatigue, or a sore mouth or tongue.

When a person has symptoms suggesting nerve damage or impairment (neuropathy), such as, tingling, burning, or numbness in their hands, arms, legs, and or/feet, a B12 test may be requested to help determine the cause.

B12 and folate testing may sometimes be ordered as part of a general health evaluation when a person shows signs of or has a history of malnutrition or malabsorption. When a breastfed infant has a 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.

When someone is being treated for a B12 or folate deficiency, he may occasionally be monitored to evaluate the effectiveness of the treatment. In a person with a nutritional deficiency, this may be done as a follow-up to treatment. In a person with a condition causing a chronic deficiency, this may be part of a long-term treatment plan.

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None needed.

Vitamin B12 and folate are both part of the B complex of vitamins. These tests measure the levels of folate and vitamin B12 in the liquid portion of the blood, the serum or plasma, to detect deficiencies. The amount of folate inside the red blood cell (RBC) may also be measured — it will normally be at a higher concentration than serum or plasma folate.

Folate refers to a natural occurring form, 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. Cobalamine, or vitamin B12, is found in animal products such as red meat, fish, poultry, milk, yogurt, and eggs and is not produced in the human body. 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).

Both B12 and folate are necessary for normal RBC formation, tissue and cellular repair, and DNA synthesis. B12 is important for nerve health, while folate is necessary for cell division such as is seen in a developing fetus. A deficiency in either B12 or folate can lead to macrocytic anemia. 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 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 deficiency during early pregnancy can increase the risk of neural tube defectssuch as spina bifida in a growing fetus. B12 and folate deficiencies are most often caused by insufficient dietary intake, inadequate absorption, or by increased need as seen in pregnancies.

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 the stores have been depleted. B12 deficiencies are sometimes seen in vegans (those who do not consume any animal products) and in their breast-fed infants.

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.

Decreased levels of vitamin B12 and folate due to increased need can be seen with a variety of diseases and conditions. Increased demand for folate occurs during pregnancy, lactation, early childhood, cancers, and chronic hemolytic anemias. All pregnant women need increased amounts of folate for proper fetal development and are encouraged by their physicians to take supplements.

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?

None needed.

How is it used?

Vitamin B12 and folate are primarily ordered to detect deficiencies and to help diagnose the cause of certain anemias. One type of associated anemia is pernicious anemia, an autoimmune disease that affects the absorption of B12. This megaloblastic anemia occurs when the body produces antibodies against the gastric parietal cells or the intrinsic factor, resulting in B12 malabsorption.

Folate, B12, and an assortment of other tests may be ordered to help evaluate the general health and nutritional status of a person with signs of significant malnutrition or dietary malabsorption. This may include people with alcoholism, other liver diseases, gastric cancer, and those with malabsorption conditions such as celiac disease, tropical sprue, Crohn disease,inflammatory bowel disease, and cystic fibrosis.

B12 and folate may also be ordered to aid in diagnosis when an individual presents with an altered mental state or other behavioral changes, especially in the elderly. B12 may be ordered with folate, by itself, or with other screening laboratory tests (antinuclear antibody, CRP, rheumatoid factor, CBC and chemistry blood tests) to help establish reasons why a person shows symptoms of neuropathy.

In those treated for known B12 and folate deficiencies, these tests will be ordered occasionally to monitor the effectiveness of treatment. This is especially true in those who cannot properly absorb B12 and/or folate and must have lifelong treatment.

Other laboratory tests that make be useful are homocysteine and methymelonic acid (MMA). Homocysteine and MMA are elevated in B12 deficiency while only homocysteine is elevated in folate deficiency. This distinction is important because treating a B12-deficient patient with folate can correct the anemia but does not stop irreversible neurologic damage.

Serum B12 and folate tests are snapshots of the concentrations in the blood. Normal values may indicate that a person’s symptoms are likely due to another cause or they may reflect the fact that a person’s stores of B12 and/or folate have not yet been fully exhausted.

In a symptomatic person, decreased concentrations of B12 and/or folate indicate the presence of a deficiency, but do not necessarily reflect the severity of the anemia or associated neuropathy. Further investigation of the underlying cause of the deficiency is then pursued. Some causes of low B12 or folate include:

• Insufficient intake—Dietary deficiency of folate or B12 is uncommon in the U.S. It sometimes may be seen with general malnutrition and in vegans who do not consume any animal products, including milk and eggs. With the introduction of fortified cereals, breads, and other grain products, folate deficiency is very rare.
• Malabsorption—Both B12 and folate deficiencies may be seen with conditions that interfere with their absorption in the small intestine. These may include:
  • Celiac disease and tropical sprue
  • Bacterial overgrowth or the presence of parasites in the intestines
  • Reduced stomach acid production from long-term use of antacids or H2 proton pump inhibitors
  • Pernicious anemia, the most common cause of B12 deficiency
  • Surgery that removes part of the stomach, such as gastric bypass, or the intestines may greatly decrease absorption.
  • Pancreatic insufficiency
• Chronic alcoholism can cause B12 and/or folate deficiency due to poor intake and impaired release of B12 from dietary proteins.
• Some drugs can cause B12 deficiency, such as metformin and omeprazole, which cause B12 malabsorption and impaired release of B12 from food proteins due to decrease in gastric acids, respectively.
• Anti-seizure medications such as phenytoin can decrease folate as can drugs such as methotrexate, which blocks folate absorption and affect body metabolism and utilization of folate, respectively.
• Increased need–All pregnant women need increased amounts of folate for proper fetal development. People with cancer that has spread (metastasized) or with chronic hemolytic anemia have increased need for folate.

If a person with a B12 or folate deficiency is being treated with supplements (or with B12 injections), then normal or elevated results indicate a response to treatment.

High levels of B12 are uncommon and not usually clinically monitored. However, if someone has a condition such as chronic myeloproliferative neoplasm, diabetes, heart failure, obesity, AIDS, or severe liver disease, then they may have an increased vitamin B12 level.

If a person is deficient in both B12 and folate but only takes folic acid supplements, the B12 deficiency may be masked. The anemia associated with both may be resolved, but the underlying neuropathy will persist.

The Schilling test was once ordered fairly routinely to confirm a diagnosis of pernicious anemia as the cause of a B12 deficiency. It is rarely, if ever, ordered and has fallen from favor because it involves the administration of radioactive B12. The Schilling test has been replaced, in part, by the measurement of intrinsic factor binding antibodies and parietal cellantibodies.