Blasts are immature bone marrow cells that will eventually become red blood cells, white blood cells or coagulation platelets. Blasts are not normally found in the blood and their presence is indicative of a bone marrow disorder (myelodysplastic syndrome). This bone marrow disorder can affect three cell lines (red blood cells, white blood cells and platelets) or only one or two lines. The quantity of blasts in the blood is expressed in billions of blasts per litre of blood (x 109/litre) or by the fraction (or percentage) of white blood cells they represent.
The interpretation of the ratio of blasts in the blood is difficult and must take several factors into account, including clinical characteristics of the patient, the speed of changes to the blood count, the number of cell lines affected (one, two or three), the probable cause of the myelodysplasia when it can be identified (primary, with one or more mutations of the chromosomes, secondary to chemotherapy), its resistance to treatment, etc. Approximately one third of myelodysplasia cases have the potential of becoming leukemia.