The BCR–ABL1 transcript burden ranges across several orders of magnitude, so results are best visualised on a log scale (as in Figure 2.4). Each log-decrease in the BCR–ABL1:ABL1 ratio represents a tenfold decrease in level. For example, a 2-log decrease means a 100-fold decrease; a 3-log decrease means a 1000-fold decrease and so on. This is designated as a superscript number, so MR3 means a 3-log reduction in BCR–ABL1:ABL1 ratio. Table 2.3 presents the categories for depth of response. The deeper the response, the better the prognosis.
Figure 2.4 Example report of a patient with chronic myeloid leukaemia being monitored for disease burden based on the BCR–ABL1:ABL1 transcript ratio (measured using quantitative reverse transcriptase polymerase chain reaction [RT-qPCR]), shown by the red line. The horizontal blue line represents the threshold for major molecular remission (3-log molecular response). The green line represents the sensitivity of detection, which varies at each RT-qPCR measurement depending on the quality of the sample and practical reasons. Adapted from Cross et al., 2018.
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