PCR test results can vary from laboratory to laboratory, and they can vary in sensitivity. It is recommended that the same laboratory should be used for follow up PCR testing to provide more accurate and consistent evaluations of response to drug therapy. Doctors are now encouraging standardization so results from different labs can be used to track the same patient.
Doctors are also encouraging the use of a type of PCR called “quantitative” PCR that returns results in terms of specific percentages of leukemic cells. Other tests, called “qualitative” PCR testing merely return “positive” or “negative” results with reference to a specified sensitivity. The problem with this approach is that changes over time are not measured.
A negative finding – often defined mathematically as a 3-log reduction, a 1,000 fold reduction from baseline, -- for the BCR-ABL gene by PCR test is a very good sign for patients. However, he or she is likely to still have cells with the Philadelphia chromosome left in the body that are not detectable even by PCR. Even with one cell left, CML can relapse. It is for this reason that doctors will rarely declare that a person is cured of CML.
A small number of patients in the chronic phase of CML will relapse after being managed well for a period of time on imatinib. That’s why regular monitoring should continue. Doctors speak of a cytogenetic relapse if the percentage of cells with the Philadelphia chromosome starts going up after a period of decline according to cytogenetic tests. Similarly, a hematological relapse occurs when your blood counts become abnormal after a period of good control.
Relapse after drug therapy can sometimes be treated with a higher dose of imatinib or with newer drug therapies. Sometimes a stem cell transplant may be considered.