Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1027 page)

   Intrinsic factor (IF), or anti-intrinsic factor, intrinsic factor–blocking antibody, type 1 intrinsic factor antibody, IFAB, is a glycoprotein produced by the gastric parietal cells. It binds to, transports, and facilitates absorption from the terminal ileum of the very small amount of vitamin B
₁₂
in the diet. If there are antibodies to the parietal cells, the B
₁₂
-binding site of IF, or the binding site of IF to the ileum, the patient’s ability to absorb dietary B
₁₂
by the IF route will be reduced. Over time, the presence of these antibodies leads to a reduction in B
₁₂
stores and ultimately to vitamin B
₁₂
deficiency, the consequences of which vary. The presence of circulating autoantibodies to IF is a very specific indicator of PA. Antibodies against IF are found in approximately 50% of cases but rarely in other conditions.

   
Normal range:
negative.

   Use

   Diagnosis of PA

   Evaluation of patients with decreased vitamin B
₁₂
levels

   Interpretation

   Increased in PA

   Limitations

   Cyanocobalamin may give a false-positive test result.

   Methotrexate and folic acid may give false-positive test results.

   Negative or inconclusive test results do not exclude the diagnosis of PA.

   Some patients with other autoimmune diseases may have positive test results, particularly in patients with autoimmune thyroid disease or type 1 DM.

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   Definition