American Board of Internal Medicine (ABIM) Certification Practice Exam

In diagnosing thrombotic thrombocytopenic purpura (TTP), which laboratory finding confirms the diagnosis?

1. Elevated platelet count

2. Presence of schistocytes in blood smear

3. Increased fibrinogen levels

4. Low hemoglobin levels

The correct answer is: Presence of schistocytes in blood smear

The presence of schistocytes in a blood smear is the laboratory finding that confirms the diagnosis of thrombotic thrombocytopenic purpura (TTP). Schistocytes are fragmented red blood cells that occur due to mechanical destruction as blood cells pass through the partially occluded microvasculature, which is characteristic of TTP. This destruction is a result of the formation of microthrombi caused by a deficiency in the von Willebrand factor-cleaving protease. While other laboratory findings may be observed in TTP, schistocytes are specifically indicative of microangiopathic hemolytic anemia associated with conditions like TTP. Increased platelet count, increased fibrinogen levels, and low hemoglobin levels, while they may present in some aspects of the disease or related conditions, do not directly confirm the diagnosis of TTP as unequivocally as the presence of schistocytes does. Thus, identifying schistocytes in a blood smear is critical for accurate diagnosis and helps guide further management of the patient.