Schilling and Protein-bound Cobalamin Absorption Tests Are Poor Instruments for Diagnosing Cobalamin Malabsorption

Objectives: To assess the advantage of a protein-bound cobalamin absorption test (PBAT) over the Schilling test in patients with suspected cobalamin (vitamin B12) malabsorption.

Design: Clinical study of consecutive patients referred from primary care units, medical and neurological clinics.

Setting: The catchment area of Sahlgrenska University Hospital, Göteborg.

Subjects: Referred patients (n = 155) with suspected cobalamin deficiency and at least one serum cobalamin value < 200 pmol L-1.

Interventions: All patients were investigated with upper gastrointestinal endoscopy with biopsies taken from the gastric body and duodenal mucosa. Serum methylmalonic acid (MMA) and homocysteine (Hcy) were determined in all 109 patients not on cobalamin substitution. A dual isotope cobalamin absorption test was then performed with the concomitant administration of crystalline (Schilling) and protein-bound cobalamin (PBAT).

Main Outcome Measures: Number of patients with gastric body atrophy diagnosed with each absorption test and the relation between these results and functional cobalamin deficiency defined as elevated MMA and Hcy, that normalized after cobalamin substitution treatment.

Results: The majority of patients with abnormal absorption tests had already developed elevated MMA and/or Hcy. PBAT was more sensitive than the Schilling test in identifying patients with gastric body atrophy but the sensitivity was too low for clinical use. About 1/3 of the patients with gastric body atrophy and normal absorption tests had elevated MMA and/or Hcy, indicating cobalamin deficiency.

Conclusion: PBAT may be somewhat more sensitive than the Schilling test but neither test is sensitive enough for diagnosing cobalamin malabsorption at an early stage.