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Pernicious anemia with false normal vitamin B12 levels caused by intrinsic factor antibodies interference: a case report Volume 81, issue 5, September-October 2023

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Authors
1 Service d’Hématologie biologique, Hôpital Européen Georges Pompidou, 20 rue Leblanc, 75015 Paris, France
2 Service de Biochimie, Hôpital Européen Georges-Pompidou, 20 rue Leblanc, 75015 Paris, France
3 Service de Biochimie, Hôpital Cochin, 27 rue du Faubourg Saint-Jacques, 75014 Paris, France
4 Service de Biochimie, Hôpital Necker, 149 Rue de Sèvres, 75015 Paris, France
5 Service d’Immunologie biologique, Hôpital Européen Georges-Pompidou, 20 rue Leblanc, 75015 Paris, France
* Correspondance : L. Darnige

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.