BCR-ABL PCR Results: — What Log Reduction Numbers Mean

A small fluctuation — within one log — between tests is common and often reflects normal variation in the test itself rather than disease progressing. The laboratory, sample handling, and timing of the blood draw can all affect the result slightly. What your oncologist watches for is a consistent upward trend across multiple tests, not a single higher reading. If your result has risen and you are concerned, raise it at your next appointment rather than waiting. Your team will decide whether the change sits within expected variation or whether testing frequency should increase.

Yes, and this matters when comparing results across laboratories or over time. A laboratory that can detect BCR-ABL down to MR5.0 sensitivity will report 'detectable' on a sample that a less sensitive test would call 'undetectable'. This is why ELN guidance emphasises testing at the same accredited laboratory wherever possible, and why a change of laboratory can appear to change your result even when your disease has not changed. If your blood is moving to a new laboratory, tell your oncologist so any apparent change can be interpreted in that context.

ELN recommends PCR testing every three months until a major molecular response is confirmed, then every three to six months once that response is stable. Frequency may be adjusted if results change, if a treatment break is being considered, or if treatment is switched. Monitoring does not stop once a deep molecular response is reached — in fact it often becomes more attentive at that point, because detecting any rise early matters most when treatment-free remission is being considered or has already begun.

An undetectable result is encouraging, but it is not on its own a reason to stop imatinib. Stopping treatment requires a careful, planned decision based on how long the deep molecular response has been sustained, the type of CML, and your full treatment history — not a single test. Stopping without that assessment carries a real risk of the BCR-ABL level rising again, which would require restarting. If treatment-free remission is something you want to explore, ask your oncologist whether your history meets the criteria in current ELN or NCCN guidance.

The International Scale (IS) is a standardisation method that allows BCR-ABL results from different laboratories to be compared on the same basis. Before the IS was introduced, a major molecular response might be defined differently by different centres, making meaningful comparison impossible. With IS standardisation, a result of 0.1% IS means the same thing whichever accredited laboratory ran the test. When your report says IS%, it is confirming this standardisation has been applied to your result.

Many patients on imatinib maintain a stable MR3.0 for years without ever reaching MR4.5, and that is a legitimate treatment outcome. MR3.0 sustained over time is associated with a low risk of disease progressing, and continuing imatinib at that level is a reasonable long-term plan. MR4.5 matters primarily if treatment-free remission is a goal you and your oncologist are actively working toward. If it is not a current goal, the focus is on keeping the response stable and monitoring for any change in trend.