The availability of TKIs offers to the patient the possibility of reaching deep and sustained molecular responses and, eventually, being part of discontinuity protocols. In some cases, the molecular response is so intense that it is not possible to detect transcripts even with a technique as sensitive as RT-qPCR; however, it is not possible to assume that the leukemic clone has disappeared, but that the remaining amount of tumor cells is below the sensitivity level of the method (residual disease). Therefore, the term complete molecular response (CMR) has been replaced by molecular response (RM), to which the level of sensitivity reached is added. The sensitivity of the test is variable from center to center, and also within the same center, from sample to sample.
Thus, we defined an MR4.0 (0.01% IS, 4 logs of reduction), MR4.5 (0.0032% IS, 4. logs of reduction) and MR5.0 (0.001% IS, 5 logs of reduction) and the MMR (major molecular response), based on the 0.1% IS limit (3 reduction logs with respect to the IRIS base value, MR3.0). In the case of “negative” or “undetectable” samples, the number of copies of the control gene marks the sensitivity with which we can rule out the presence of BCR-ABL1; MR4.0 when ABL> 10,000 copies, MR4.5 when ABL> 32,000 copies, and MR5.0 when ABL> 100,000 copies (see Table 1).