A 25-year-old patient complains about weakness, dizziness, appearance of haemorrhagic skin rash. She has been suffering from this for a month. Blood count: erythrocytes: 1,0 × 1012/l, Hb- 37 g/l, colour index - 0,9, leukocytes - 1,2 × 109/l, thrombocytes - 42 × 109/l. What diagnostic method will be the most effective?