Intrinsic factor antibody

The expected result is Negative; some laboratories print a number in U/mL or AU/mL with a threshold, and the printed positive or negative conclusion is the result that matters.

Intrinsic factor is the protein made by the stomach lining that vitamin B12 must attach to before it can be absorbed. In pernicious anaemia the immune system makes antibodies against intrinsic factor, so B12 cannot be absorbed however much is eaten. The test looks for those antibodies in the blood.

Doctors request it when vitamin B12 is low, to find out whether pernicious anaemia is the cause, because that decides whether B12 is given by injection for life rather than by mouth. A positive result settles the question; a negative one does not exclude it, since around half of people with pernicious anaemia test negative.

• Pernicious anaemia, an autoimmune condition of the stomach lining
• Other autoimmune conditions, such as autoimmune thyroid disease, occasionally

• A negative result means the antibody was not found; it does not exclude pernicious anaemia

• A recent B12 injection can produce a false positive with some laboratory methods, so the test is best taken before an injection or at least a week after
• A negative result is found in about half of people who do have pernicious anaemia, so it is a one-way test
• A weak positive close to the threshold is sometimes repeated

No fasting needed. Best taken before any B12 injection, or at least a week afterwards.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.