INTRINSIC FACTOR ANTIBODIES - serum

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Specimen: 5 mL blood in plain tube. As recent vitamin B12 administration is associated with a high rate of false positive results, the sample must be collected prior to commencing therapy or at least one week after vitamin B12 administration.

Method: Immunoassay.

Reference Interval: Consult pathologist.

Application: Diagnosis of pernicious anaemia.

Interpretation: High specificity for pernicious anaemia and a positive result is sufficient to establish the diagnosis. Sensitivity is low, thus a negative result does not exclude the diagnosis of pernicious anaemia. Type 1 (blocking) antibody prevents the attachment of vitamin B12 to intrinsic factor: present in 50-60% of patients with pernicious anaemia. Type 2 (precipitating) antibody prevents attachment of the vitamin B12-intrinsic factor complex to ileal receptors: present in 30% of patients with pernicious anaemia, and only in those who also have Type 1 antibodies. Immunoassay kits which rely on blocking function of antibodies are usually less sensitive.

Reference: Chanarin I. The Megaloblastic Anaemias. 3rd ed. Blackwell Scientific 1990. Carmel R. Clin Exp Immunol 1992; 89: 74-77.

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