Monitoring and defining early response: Where to draw the line?

Over recent years the early response to therapy has been established as a robust and critical determinant of long term outcome to tyrosine kinase inhibitor therapy. Molecular monitoring has taken centre stage with the incorporation of molecular milestone values into treatment recommendations and guidelines. However, establishing a reliable molecular method that is standardized to the international reporting scale is not a trivial undertaking and more recent data suggest that a single timepoint molecular assessment may not be optimal for identifying the patients at highest risk of treatment failure. This review will discuss the evidence for the importance of the early assessment of response for treatment change decisions, the emerging evidence for incorporating additional sample collection timepoints to assess the initial rate of BCR-ABL1 decline and the controversial suggestion that methods be changed to accommodate this analysis.