A 58-year-old woman complains of spontaneous bruises, weakness, bleeding gums, dizziness. Objectively: the mucous membranes and skin are pale with numerous hemorrhages of various time of origin. Lymph nodes are not enlarged. Ps- 100/min, BP-110/70 mm Hg. There are no alterations of internal organs. Blood test results: RBC-3,0 × 1012/l, Hb- 92 g/l, color index - 0,9, anisocytosis, poikilocytosis, WBC- 10 × 109/l, eosinophils - 2%, stab neutrophils - 12%, segmented neutrophils - 68%, lymphocytes -11%, monocytes - 7%, ESR - 12 mm/h. What laboratory test should be performed additionally to make the diagnosis?