Intrinsic Factor Antibody

Anemia

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Autoantibody test.

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Intrinsic Factor Antibody Test Information Guide

Why is it done?
- Detects autoantibodies against intrinsic factor, a protein produced by gastric parietal cells necessary for vitamin B12 (cobalamin) absorption in the terminal ileum
- Diagnoses autoimmune pernicious anemia, a condition where the immune system attacks gastric parietal cells, leading to vitamin B12 deficiency and megaloblastic anemia
- Investigates unexplained vitamin B12 deficiency or macrocytic anemia when parietal cell autoimmunity is suspected
- Evaluates patients with neurological symptoms suggesting B12 deficiency (peripheral neuropathy, cognitive changes, weakness)
- Typically ordered when serum B12 levels are low and malabsorption is suspected as the underlying cause
- Performed during initial diagnostic workup or when confirming suspected pernicious anemia diagnosis
Normal Range
- Result: Negative or <1.0 IU/mL (International Units per milliliter)
- Units: IU/mL (International Units per milliliter) or presence/absence (qualitative)
- Normal Result Interpretation: Negative (no detectable antibodies) indicates absence of autoimmune attack against intrinsic factor; pernicious anemia unlikely if B12 deficiency is present
- Abnormal Result Interpretation: Positive (≥1.0 IU/mL or antibodies detected) is highly specific for autoimmune pernicious anemia
- Borderline/Equivocal Range: Some laboratories may report results in a borderline range (0.9-1.1 IU/mL) requiring clinical correlation and possible repeat testing
Interpretation
- Positive Result (≥1.0 IU/mL):
  - Highly specific (95-98% specificity) for autoimmune pernicious anemia diagnosis
  - Confirms that vitamin B12 malabsorption is due to autoimmune destruction of intrinsic factor-secreting cells
  - Indicates lifelong need for B12 supplementation via intramuscular injections
- Negative Result (<1.0 IU/mL):
  - Makes autoimmune pernicious anemia unlikely; suggests alternative cause of B12 deficiency or malabsorption
  - May indicate dietary B12 insufficiency, gastrointestinal disorders (Crohn's, celiac disease), or post-gastrectomy status
  - Sensitivity is lower (50-70%); some patients with pernicious anemia may be IF antibody negative
- Factors Affecting Results:
  - Timing of testing: Antibodies may not be detectable early in disease; may appear or disappear during treatment
  - Active immunosuppressive therapy may reduce antibody levels or produce negative results
  - Recent blood transfusions or immunoglobulin administration may affect test accuracy
  - Comorbid autoimmune conditions may be associated with positive results
- Clinical Significance:
  - Most specific test for pernicious anemia; positive result essentially confirms diagnosis when combined with low B12 and elevated methylmalonic acid/homocysteine
  - Can be positive years before symptomatic vitamin B12 deficiency manifests (early marker of autoimmune disease)
  - Often accompanies other autoimmune phenomena (gastric parietal cell antibodies, thyroid antibodies)
Associated Organs
- Primary Organ System:
  - Gastrointestinal system: Specifically the stomach (parietal cells of fundus and body produce intrinsic factor)
  - Hematopoietic system: Bone marrow where vitamin B12 is required for DNA synthesis and red blood cell maturation
- Associated Medical Conditions:
  - Pernicious anemia (autoimmune type B12 deficiency) - primary diagnosis
  - Atrophic gastritis (chronic inflammation of stomach lining leading to loss of parietal cells)
  - Autoimmune thyroid disease (Hashimoto's thyroiditis, Graves' disease) - often coexist
  - Type 1 diabetes mellitus - shared autoimmune tendency
  - Addison's disease (autoimmune adrenalitis)
  - Systemic lupus erythematosus (SLE) and other autoimmune connective tissue diseases
- Complications and Risks of Abnormal Results:
  - Megaloblastic anemia: Abnormal, immature red blood cells leading to fatigue and dyspnea
  - Subacute combined degeneration of spinal cord: Permanent neurological damage (progressive myelopathy, ataxia, loss of proprioception)
  - Peripheral neuropathy: Distal sensory loss, paresthesias, potential permanent damage if untreated
  - Cognitive dysfunction and dementia: Memory loss, confusion, potential irreversible cognitive decline
  - Increased risk of gastric adenocarcinoma: Chronic atrophic gastritis is a precancerous condition
  - Cardiac arrhythmias: From severe anemia or elevated homocysteine levels
Follow-up Tests
- Confirmatory Tests (if positive):
  - Parietal cell antibodies: Positive in 80-90% of pernicious anemia patients; supports autoimmune diagnosis
  - Serum vitamin B12 level: Quantifies degree of B12 deficiency; guides treatment decisions
  - Methylmalonic acid (MMA) and homocysteine: Elevated in B12 deficiency; more sensitive than B12 alone
  - Complete blood count (CBC): Shows macrocytic anemia, hypersegmented neutrophils, low platelets if severe
  - Peripheral blood smear: Shows megaloblastic changes confirming B12-dependent anemia
- Additional Diagnostic Tests:
  - Schilling test (historical): Evaluates B12 malabsorption pattern; less commonly performed now
  - Upper endoscopy: May reveal atrophic gastritis; allows biopsy for assessment of chronic inflammation
  - Gastric pH assessment: Evaluates achlorhydria (absence of stomach acid) associated with pernicious anemia
- Related Autoimmune Screening:
  - Thyroid function tests (TSH, free T4) and anti-TPO antibodies: Screen for coexisting thyroid autoimmunity
  - Tissue transglutaminase (tTG) IgA: Screen for celiac disease which can cause B12 malabsorption
  - Antinuclear antibody (ANA): Screen for other autoimmune connective tissue diseases
- Monitoring and Follow-up Schedule:
  - Initial monitoring: CBC and B12 levels 1-3 months after starting B12 treatment
  - Ongoing monitoring: Vitamin B12 levels and CBC annually or per treatment protocol; adjust B12 injections based on levels and symptoms
  - Symptom assessment: Regular evaluation for peripheral neuropathy and neurological symptoms
  - Intrinsic factor antibody: Not typically repeated; remains positive once established
  - Gastric cancer screening: Periodic endoscopy may be considered due to increased malignancy risk
Fasting Required?
- Fasting Requirement: No
- Intrinsic factor antibody testing does not require fasting; the test can be performed at any time of day regardless of food intake
- Special Patient Preparation Instructions:
  - No special dietary restrictions needed before blood draw
  - Normal medications may be continued; inform phlebotomist of all current medications
  - Continue regular B12 supplementation if already taking; note timing relative to blood draw if possible
  - If possible, avoid scheduling blood draw immediately after B12 injection for most accurate antibody measurement
  - Immunosuppressive therapy should be noted on requisition as it may affect results
  - Standard venipuncture with serum separator tube (SST) or similar collection tube per laboratory protocol
  - Inform laboratory if any recent blood transfusions or immunoglobulin administration has occurred

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