A 48-year-old man complains of fatigue, excessive sweating, severe skin itching, undulant fever, enlarged cervical and supraclavicular lymph nodes. Objectively: paleness of skin and mucosa, cervical lymph nodes are mobile, dense, elastic, walnut-sized, painless, not attached to the skin. Complete blood count: erythrocytes - 3,0 × 1012/l, Hb-100 g/l, leukocytes - 14 × 109/l, eosinophils -6%, basophils - 3%, band neutrophils -11%, segmented neutrophils - 69%, lymphocytes -7, monocytes - 4%, platelets - 280 × 109/l, ESR - 37 mm/hour. What method should be applied to verify the diagnosis?