Intrinsic Factor Antibody
A blood sample drawn from a vein in your arm
You should wait at least 48 hours after receiving a vitamin B12 injection to have a blood sample taken. Some healthcare providers may prefer not to draw a blood sample for the test within 2 weeks of an injection.
Intrinsic factor is a protein produced by a type of specialized cells that line the stomach wall known as parietal cells. During digestion, stomach acids release vitamin B12 from food and bind to intrinsic factor to form a complex. The formation of this complex is necessary for the absorption of vitamin B12 in the small intestine.
Among having functional roles in the brain and nervous system, vitamin B12 is important in the production of red blood cells. Without sufficient intrinsic factor, vitamin B12 goes largely unabsorbed and the body cannot produce enough normal red blood cells, leading to anemia. Besides anemia, decrease in the numbers of neutrophils and platelets (neutropenia, thrombocytopenia) may also occur.
Anemia that is due to a lack of intrinsic factor is called pernicious anemia. This is primarily an autoimmune condition that occurs when the body's immune system targets its own tissues and develops antibodies directed against the parietal cells and/or the intrinsic factor. These antibodies can damage the parietal cells and disrupt intrinsic factor production or prevent intrinsic factor from carrying out its biological function.
Two types of IF antibodies can be tested by the laboratory:
- Intrinsic factor blocking antibody (type 1), which is more specific for pernicious anemia and is the one that is usually tested.
- Intrinsic factor binding antibody (type 2), which interferes with the uptake of the vitamin B12-instrinsic factor complex.
How is it used?
Pernicious anemia is caused by vitamin B12 deficiency due to a lack of intrinsic factor. This condition occurs primarily when the body's immune system targets its own tissues and develops antibodies directed against the parietal cells and/or the intrinsic factor. These antibodies can damage the parietal cells and disrupt intrinsic factor production or prevent intrinsic factor from carrying out its biological function. Intrinsic factor binds to vitamin B12, forming a complex and allowing its absorption in the small intestine. (For more, see the "What is being tested?" and the condition article on Vitamin B12 Deficiency.)
IF antibody testing is usually used in follow up after other laboratory tests, such as a vitamin B12 test, a methylmalonic acid test and a complete blood count (CBC), have established that a person has a vitamin B12 deficiency and associated anemia and/or neuropathy.
It may be used in conjunction with a test for parietal cell antibodies to aid in establishing a diagnosis.
There are two types of IF antibodies that may be tested:
- Intrinsic factor blocking antibody (type 1), which blocks the binding site of vitamin B12 to intrinsic factor. This is more specific for pernicious anemia and is the one that is usually tested.
- Intrinsic factor binding or precipitating antibody (type 2), which interferes with the uptake of the vitamin B12-instrinsic factor complex in the small intestine.
When is it ordered?
The intrinsic factor antibody (IF antibody) test is not a frequently ordered test. It may be ordered when a person has a vitamin B12 deficiency and when pernicious anemia is suspected, as when a person has signs and symptoms such as:
- Weakness, fatigue
- Numbness and tingling in the feet and/or hands
- Larger than normal red blood cells (RBCs); sometimes large RBCs are detected before the other signs and symptoms emerge, for example, with a complete blood count (CBC) during routine testing for a health examination.
When a person has a decreased vitamin B12 level and increased methylmalonic acid and homocysteine levels, an IF antibody test is typically ordered.
What does the test result mean?
The results of intrinsic factor antibody (IF antibody) tests are often taken into consideration with the results of other laboratory tests to help make a diagnosis. When a person has a decreased vitamin B12 level and/or increased methylmalonic acid and homocysteine levels and has IF antibodies, then it is likely that the person has pernicious anemia.
A negative test result does not necessarily mean that a person does not have pernicious anemia. As many as half of those affected will not have IF antibodies. When they are not present, the health practitioner may order a parietal cell antibody test to help establish the diagnosis. Parietal cell antibodies are not as specific as IF antibodies. They are present in about 90% of those with pernicious anemia but may also be present in a variety of other conditions and in up to 10% of the general population.
Should everyone have an intrinsic factor antibody test?
Can I have an intrinsic factor antibody test done at my doctor's office?
Can I get rid of intrinsic factor antibodies?
Is there anything else I should know?
Some people with autoimmune diseases such as type 1 diabetes, Hashimoto thyroiditis, Addison disease or Graves disease may have IF antibodies without having pernicious anemia. The IF antibody test is not used to diagnose or monitor these conditions.
Vitamin B12-associated anemia may take several years to develop as a normal person typically has large stores of B12 in reserve. Symptoms tend to emerge only when these stores become depleted.