A 30-year-old woman presents with recurrent petechiae, ecchymoses, and menorrhagia for 6 months. Physical exam shows mild anemic appearance, non-palpable liver/spleen, petechiae, and multiple ecchymoses on lower limbs. CBC reveals Hb 86g/L (anemia) and WBC 6.5×10<sup>9</sup>/L. Which additional test should the outpatient doctor order?