Morphologic diagnosis of thrombotic thrombocytopenic purpura

The diagnosis of thrombotic thrombocytopenic purpura (TTP) rests on evidence of microangiopathic hemolytic anemia and thrombocytopenia in the absence of disseminated intravascular coagulation and other known causes of thrombotic microangiopathy. Highly specific diagnostic tools such as serum levels of ADAMTS13 are not routinely available for immediate clinical diagnosis. The presence of schistocytes on a blood smear is the morphologic hallmark of the disease, but no guidelines exist as to the number of schistocytes required to differentiate TTP from other thrombotic microangiopathies. We studied 6 patients with TTP and compared their schistocyte counts with those of 40 normal subjects, 28 patients with chronic renal disease, 5 with preeclampsia, and 5 with normal functioning mechanical heart valves. The mean schistocyte count for the TTP patients was 8.35% versus 0.05% for normal subjects, 0.2% for renal patients, 0.25% for preeclamptic patients, and 0.18% for patients with mechanical valves (P < 0.001). Schistocytes were found on 100% of blood films of TTP patients and ranged from 1.0% to 18.4% of red cells. Schistocytes are found on the smears of 58% of normal individuals and on 80-100% of the other patient groups studied, but always comprise less than 0.5% of the red cell population. An initial schistocyte count of greater than 1% strongly suggests a diagnosis of TTP in the absence of other known causes of thrombotic microangiopathy.