The level of the BCR-ABL1 fusion transcript is expressed as a percent of the ABL1 gene transcript. For standardization purposes, the P210 transcript ratio is further normalized to the international scale (IS) and reported as BCR-ABL1/ABL1%. The IS provides a standardized correction factor that enables direct comparison of results from different laboratories, regardless of differences in methodology.1,2 No IS currently exists for the P190 isoform, so P190-positive cases are reported as BCR-ABL/ABL1% with no further normalization.
To assess molecular response to therapy, results are compared against a “universal baseline” BCR-ABL1 ratio. A value of 100% indicates no response. Values of 10%, 1%, 0.1%, 0.01%, 0.0032%, and 0.001% correspond to decreases of 1, 2, 3, 4, 4.5, and 5 logs, respectively, relative to the universal baseline of 100%. A BCR-ABL1 expression of ≤0.1% corresponds to a major molecular response (MMR).3