During your initial diagnosis, your doctor will likely order cytogenetic testing to check for the presence of the Philadelphia chromosome, a specific genetic abnormality that is found in bone marrow and in blood cells of patients with CML. As leukemia appears, these irregular cells increase in number as compared to normal white blood cells.
Cells are removed from the marrow and analyzed for the presence of the Philadelphia chromosome. Marrow is chosen because it is here that blood cells are dividing, and it is at this stage in the life cycle of cells that their chromosomes can be seen via a microscope. Twenty or thirty blood cells are analyzed. In some cases, especially before treatment when white blood counts may be extremely high, cytogenetic testing can be performed on sample of blood instead of marrow.
Spotting only one cell with the Philadelphia chromosome is enough to confirm a diagnosis of CML, but often, at the time of diagnosis, all twenty or thirty cells contain the Philadelphia chromosome. As successful therapy continues, your doctor will track reductions in the percentage of these cells.
The degree to which your body is responding to treatment is described in percentages. Any reduction in the percentage of cells containing the Philadelphia chromosome is considered a cytogenetic response. If the number of leukemic cells with the Philadelphia chromosome falls to fewer than 35 percent, that is considered a major cytogenetic response. If cytogenetic testing reveals that there are no more cells with the Philadelphia chromosome, or zero percent, that is considered a complete cytogenetic response.