MRD, or minimal residual disease, refers to the detection of remaining cancer cells that are so sparse they evade standard detection methods. Tests for minimal residual disease can identify early tumors and, in treated patients, the presence of MRD suggests that treatment was not fully effective. Identifying MRD helps determine whether a patient requires more aggressive and potentially harmful treatment. The basic concept of MRD is that its identification can guide clinical decisions and potentially improve cure rates.Several methods exist for detecting MRD, including flow cytometry and PCR (polymerase chain reaction). These techniques are capable of identifying a small number of abnormal cells among a large population of normal ones.Flow Cytometry: This laser-based technique analyzes cell samples, such as blood, to identify and count malignant cells. Cells are tagged with fluorescent antibodies that bind to specific antigens expressed by cancer cells, allowing the flow cytometer to detect them. This method can identify a single leukemic cell amidst 10,000 or more normal cells.PCR: This method detects cancerous cells by identifying their unique chromosomal rearrangements. For instance, PCR can reveal the presence of the Philadelphia (Ph) chromosome, which is present in about 95% of chronic myelogenous leukemia (CML) patients, 2%-10% of children with acute lymphoblastic leukemia (ALL), and 20%-50% of adults with ALL. The Ph chromosome results from a translocation between chromosomes 9 and 22, creating a bcr-abl fusion gene that drives the disease. PCR can detect the bcr-abl mRNA transcript, identifying one Ph-positive cell among a million normal ones.
