A 60-year-old man complains of dyspnea, paresthesia, vertigo, and fatigability. He has a history of atrophic gastritis. Objectively, his skin and mucosa are pale and icteric. Lingual papillae are smoothed out. The liver and pancreas are not enlarged. Complete blood count shows the following: Hb — 77 g/L, erythrocytes — 1,65 •109/L, color index — 1.4, reticulocytes — 0.2%, leucocytes— 2.8 • 109/L; ESR — 22 mm/hour, macrocyte. What additional test can reveal the cause of anemia?