Although peripheral destruction of red cells, leukocytes, and platelets may occur together and lead to clinically significant pancytopenia, depression of all three cell lines also suggests
bone marrow failure, as in the case in aplastic anemia. Thus, bone marrow examination is the most important diagnostic test to perform. Causes of marrow failure include drugs and coincidental diseases including: the acute leukemias, large granular lymphocyte leukemia, the myelodysplastic syndromes, marrow replacement by fibrosis or tumor, severe megaloblastic anemia, paroxysmal nocturnal hemoglobinuria (PNH), and overwhelming infection. In addition, unexplained cytopenia can be associated with bone marrow necrosis, dysplasia, and distortion of the bone marrow architecture