[Pernicious anemia with false normal vitamin B12 levels caused by intrinsic factor antibodies interference: a case report].

We present a case of a 48-year-old woman with a fortuitous discovery of macrocytic anemia and thrombocytopenia. Serum folate and vitamin B12 levels were normal. However, due to the presence of indirect signs of cobalamin deficiency, such as elevated homocysteine and methylmalonic acid, and signs of dyserythropoiesis on the bone marrow aspirate, pernicious anemia was suspected. Vitamin B12 dosage was repeated finding fluctuating but always normal results. Anti-intrinsic factor antibodies were present at a very high level, explaining the fluctuations and the interference found on the assay using competitive binding chemiluminescence (CBLA). Serum vitamin B12 dosage by electrochemiluminescence, a method described as not interfering with intrinsic factor antibodies, showed a collapsed vitamin B12 level. Measurement of vitamin B12 with CBLA after adsorption of immunoglobulins in the sample using protein G SepharoseTM, confirmed the interference of the cobalamin assay with autoantibodies. This case illustrates the difficulties regarding the analysis and standardization of the vitamin B12 assay for the diagnosis of pernicious anemia.