Classification of severity
The severity of children AA was classified by the following criteria[17]. SAA with: 1) bone marrow karyocyte count <25% or 25%-50% with <25% hematopoietic elements; 2) peripheral blood with at least two of the following factors, including granulocytes <0.5×109/L, platelets <20×109/L, reticulocytes <0.2×109/L or corrected reticulocyte count <1%. Very severe aplastic anemia (VSAA) presenting same as SAA,except for granulocytes <0.2×109/L.