A 22-year-old female patient presents with menorrhagia for 8 months, gingival bleeding for 2 weeks, and scattered petechiae and ecchymoses on the lower limbs. Outpatient tests show hemoglobin 80 g/L, WBC 5.2×10<sup>9</sup>/L, and platelets 32×10<sup>9</sup>/L. Gynecological examination is unremarkable. Which of the following laboratory findings is direct evidence for diagnosing idiopathic thrombocytopenic purpura (ITP)?