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Blood Test Tracker

Intrinsic Factor Blocking Antibody

Looks for antibodies that block intrinsic factor, a stomach protein needed to absorb vitamin B12; a positive result supports pernicious anemia.

AI-drafted. Awaiting verificationAlso called IFBA, IFAB, IF AbLOINC 31443-5

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What it is

Intrinsic factor is a protein made in the stomach. It attaches to vitamin B12 so the small intestine can absorb it. In some people the immune system makes antibodies that block this step. This test looks for those antibodies, called blocking or type 1 antibodies. It is usually ordered when a vitamin B12 level is low and the cause is unclear.

Why it matters

Blocking antibodies are strongly linked to pernicious anemia, an autoimmune cause of vitamin B12 deficiency. A positive result is uncommon in people who do not have pernicious anemia, but only about half to 70% of people who have it test positive. So a positive result supports the diagnosis, while a negative result does not rule it out. Finding an autoimmune cause matters because it is a lasting one, and it can change how your clinician plans long-term follow-up.

If it’s high

A positive result means antibodies that block intrinsic factor were found. This strongly supports autoimmune gastritis, the usual cause of pernicious anemia. It is not a diagnosis by itself, and your clinician will read it with your vitamin B12 level, blood count and symptoms. A vitamin B12 injection shortly before the blood draw can cause a false positive.

Common causes

  • A vitamin B12 injection given shortly before the blood draw, which can cause a false positive
  • Autoimmune gastritis, which can lead to pernicious anemia

What can help

  • Tell your clinician if you had a vitamin B12 injection before the blood draw
  • Ask how this result fits with your vitamin B12 level, blood count and symptoms
  • Ask whether other follow-up tests, or a stomach check, make sense

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–1.1 AU/mL. Source: Northwell Health Laboratories.

What can affect your result

  • Vitamin B12 injections before the blood draw can cause a false positive
  • Disease stage: the test is more likely to be positive later in the disease (one review reports sensitivity rising to about 80% as it progresses)
  • Kit and method: blocking and binding antibody tests differ, and four methods disagreed on 35% of samples in one study
  • A recent vitamin B12 injection can cause a false-positive result, so the blood should be drawn before B12 is given. Labcorp's collection note says not to collect a sample less than one week after the last B12 injection.
  • This test finds only one kind of antibody (the blocking, or type 1, kind). A negative result does not rule out autoimmune gastritis.
  • Kits differ in what they detect (blocking or binding antibodies) and in their cutoffs, and different methods can disagree on the same sample.
  • ARUP rejects grossly hemolyzed or severely lipemic samples.

When to talk to a clinician

Contact a clinician soon if you have numbness or tingling in your hands or feet, trouble with balance, memory changes, a sore smooth tongue, or unusual tiredness or paleness, especially with a low vitamin B12 result. Call 911 or your local emergency number for chest pain, trouble breathing, fainting or confusion.

Questions you could ask

  • If this test is negative but my vitamin B12 stays low, what else should we look at?
  • Which type of intrinsic factor antibody does my lab test for?
  • Does this result fit with my vitamin B12 level and blood count?
  • Should we check my stomach or other follow-up tests?
  • Could a recent vitamin B12 injection have affected this result?

Questions people ask

Does a negative result rule out pernicious anemia?

No. Only about half to 70% of people with pernicious anemia have these antibodies, and NICE notes that a negative result does not rule out autoimmune gastritis. If your clinician still suspects it, other tests such as parietal cell antibodies or gastrin can help.

Why was this ordered after my low vitamin B12?

A low B12 has several possible causes. This test looks for one autoimmune cause. Knowing the cause helps your clinician decide how long B12 treatment may be needed and what follow-up makes sense.

Will a recent vitamin B12 shot change the result?

It can. A B12 injection before the blood draw can cause a false-positive result, so clinicians try to draw the blood first. Tell your clinician if you have had any B12 injections.

Show the science and sources

For clinicians and researchers

Intrinsic factor (IF) antibodies come in two kinds. Type 1 (blocking) antibodies stop B12 from binding IF, and type 2 (binding) antibodies interfere with the IF-B12 complex binding to receptors in the ileum. Type 1 is found in about 70% of people with pernicious anemia and type 2 in 35 to 40%, often together. Specificity is almost 100%, and sensitivity is 50 to 70% (37% in one ELISA study), rising to about 80% as the disease progresses. Parietal cell antibodies are more sensitive (80 to 90%) but less specific. IF antibody results can be spuriously positive after cobalamin injections, so blood should be drawn before B12 is given. NICE NG239 (2024) advises considering an anti-IF antibody test in people with B12 deficiency when autoimmune gastritis is suspected and they have not had a positive test before or surgery that affects B12 absorption. It states that a negative result does not rule out autoimmune gastritis, that labs should say what their assay detects, and that further steps after a negative test include parietal cell antibodies, gastrin, a CobaSorb test and gastroscopy with biopsy. ARUP's assay is a qualitative ELISA reported as negative or positive, and a Mayo-based send-out listing reports negative, positive or indeterminate. Labcorp's assay is an immunochemiluminometric assay (ICMA) that reports AU/mL with a reference interval of 0.0 to 1.1. A comparison of four methods (one competitive automated and three manual non-competitive assays) found discordant results in 35% of 62 sera, although each method found some proven cases the others missed. A published study using the Inova QUANTA Lite ELISA called 25 units or more positive, 20.1 to 24.9 equivocal and 20 or less negative.

LOINC 31443-5 (primary), 9537-2, 31444-3, 31445-0. Specimen: serum.

Sources

  1. Intrinsic Factor Blocking Antibody (ARUP test 0070210), ARUP Laboratories (2026)
  2. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239), National Institute for Health and Care Excellence (NICE) (2024)
  3. Pernicious Anemia: Basic Pathophysiology and Diagnostic Challenges in Neuropsychiatric Patients, Hematology Reports 2026;18:47 (2026)
  4. The Diagnostic Value of Anti-Parietal Cell and Intrinsic Factor Antibodies, Pepsinogens, and Gastrin-17 in Corpus-Restricted Atrophic Gastritis, Diagnostics 2022;12:2784 (2022)
  5. Do intrinsic factor antibodies assays provide univocal answers in Biermer's disease?, Journal of Clinical Laboratory Analysis 2008;22:337-339 (2008)
  6. Intrinsic Factor Blocking Antibodies (Northwell Health Laboratories test 56184, Labcorp send-out), Northwell Health Laboratories
  7. Intrinsic Factor Abs, Serum (Labcorp test 010413), Labcorp (test directory)
  8. Intrinsic Factor Blocking Antibody (UW Medicine Laboratory test RINFA, sent to Mayo), UW Medicine Laboratory Medicine
  9. LOINC codes verified with the NLM Clinical Tables LOINC search (e.g. 31443-5), U.S. National Library of Medicine

AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.

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