A 58-year-old female patient 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 is 100/min, AP - 110/70 mm Hg. There are no changes of internal organs. Blood test results: RBC - 3,0 × 1012/l, Нb - 92 g/l, colour 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 is to be determined next for making a diagnosis?