A 45-year-old man complains of loss of appetite, dizziness, severe general weakness, low-grade fever. Pale skin, bruises on the torso and limbs. ZAK found: erythrocytes - 2,9 × 1012/ l, Hb - 96 g/l, KP - 0,99, reticulocytes - 0,02%, leukocytes - 2.9 × 10 ^ g/l, platelets - 85 × 10 ^ g/l, ESR - 45 mm/h. What additional research is there most appropriate for verification of the diagnosis?