Chronic myeloid leukaemia is a blood and bone marrow disease that develops slowly. It has three phases. The phase is determined by the number of blast cells in the blood and bone marrow and by the severity of symptoms.

Most people are diagnosed when the CML is in the chronic phase. At this time CML develops very slowly and is often stable for a long time. It is sometimes called the ‘stable phase’. There may be no symptoms and most people find they can lead a normal life. It is rare to need to go into hospital. You can be given treatment as an outpatient and usually it does not cause many side effects. You will have regular appointments to check your condition and to have your blood tested. Although it varies from person to person this phase lasts on average about 4–5 years, but can be longer.

In chronic phase CML there are 5% or fewer blast cells in the blood and bone marrow.

After some time, the leukaemia may gradually move into an accelerated phase, during which it develops more quickly. In this phase there are 6–30% blast cells in the blood and bone marrow. Sometimes this change may be picked up from your blood tests, in which blast cells can be seen, or from new symptoms you may have. These may include:

• feeling very tired (due to anaemia)
• infections
• bruising or bleeding.

If you notice any of these symptoms, let your doctor know straight away. The treatment for the accelerated phase is often more intensive than in the chronic phase and you may need to spend some time in hospital as an in-patient.

After some time in the accelerated phase, usually months, the leukaemia ‘transforms’ into a blast phase, which is more like an acute leukaemia. In this phase much of the bone marrow has been replaced by many very immature (blast) cells and there are more than 30% blast cells in the blood and bone marrow.

In some people chronic myeloid leukaemia changes quickly from the chronic phase to the blast phase without going through the accelerated phase. If tiredness, a high temperature and an enlarged spleen occur during the blast phase this is known as blast crisis.

Rarely, CML develops into a condition called myelofibrosis. This means that the bone marrow can't make red cells, white cells or platelets because it is replaced by scar tissue (fibrosis).

A relapse is when the number of blast cells increases after a period of remission. Remission is when the number of blast cells in the blood and bone marrow goes back to normal following treatment.

Complete remission means that all of the blood and bone marrow tests are normal. A molecular remission is when the Philadelphia chromosome can't be found even by the very sensitive PCR test.