Monitoring response to treatment in CML

During and after treatment for chronic myeloid leukaemia (CML), you have regular blood tests to monitor your response. These tests:

• Check if your blood cell counts have returned to normal levels. This is done using a test called a full blood count. It is sometimes called your haematological response.
• Measure the amount of the BCR-ABL1 gene in your blood. This is done using a test called PCR. It is called your molecular response.

You also have regular appointments to check how you are feeling. This includes any symptoms or side effects you’re getting and any emotional or mental health concerns you have.

If you have any serious or worrying symptoms, you do not have to wait for your next appointment. Your haematology team should give you details of who to contact. This may be a Clinical Nurse Specialist (CNS), if one is available in your area.

Molecular response (MR) is the amount of BCR-ABL1 gene in your blood. It is a good indicator of how much leukaemia is left in your body. It is measured using a test called PCR. This test is very sensitive so it can detect extremely low levels of leukaemia in your body.

What do molecular response results mean?

The results of your PCR tests are written as a percentage or as an MR number. They can be difficult to understand.

• MR1: Less than 1 in 10 white blood cells (10%) has the BCR-ABL1 gene. If your treatment is working well, you should reach MR1 within 3 months of starting treatment.
• MR2: Less than 1 in 100 white blood cells (1%) has the BCR-ABL1 gene. If your treatment is working well, you should reach MR2 within 6 months of starting treatment.
• MR3: Less than 1 in 1,000 white blood cells (0.1%) has the BCR-ABL1 gene. This is sometimes called a major molecular response (MMR). If your treatment is working well, you should reach MR3 within 12 months of starting treatment.
• MR4: Less than 1 in 10,000 white blood cells (0.01%) has the BCR-ABL1 gene. This is sometimes called a deep molecular response (DMR). If you reach and maintain a deep molecular response, you might eventually be able to stop treatment.
• MR5: Less than 1 in 100,000 white blood cells (0.001%) has the BCR-ABL1 gene. This is also called a deep molecular response (DMR). If you reach and maintain a deep molecular response, you might eventually be able to stop treatment.
• Levels below MR5 cannot usually be detected. This is called a complete molecular response.

We have a separate factsheet about molecular response.

If you’re not sure what your results mean, ask your haematology team to explain.

Why is molecular response important?

Your molecular response can be used to detect any difficulties in treatment early on. It is also an essential part of safe monitoring if you can stop treatment.

• Some people can have a slower response to treatment and may not necessarily reach MR3 after 12 months. If you do not reach these milestones, your haematology team will talk to you about your options. They might suggest changing your treatment. This may allow you to reach a deeper response.
• If you respond to treatment at first but your level of BCR-ABL1 starts to go back up, your haematology team will talk to you about your treatment options. They may suggest increasing your dose or changing to a different treatment.