A diagnosis of CML requires the following two conditions:

  • Persistent unexplained increase in the number of white blood cells (white cell count above 11.0´109/L)
  • Identification of the Philadelphia chromosome t(9;22) or the BCR-ABL1 fusion gene in the peripheral blood or bone marrow cells.

About 95% of CML cases exhibit a Philadelphia chromosome or a variant chromosome.

These variations may be simple, involving chromosome 22 and another chromosome other than chromosome 9.

Alternatively, they may be complex, involving one or more other chromosomes in addition to chromosomes 9 and 22 (Jabbour and Kantarjian 2020).

Diagnosis in these patients depends on being able to identify the BCR‑ABL1 fusion gene

Patients with a variant chromosome should receive the same treatment as patients with a Philadelphia chromosome t(9;22) as they respond to the same treatment.

In the remaining 5% of patients, neither the Philadelphia chromosome nor the BCR‑ABL1 gene in a variant chromosome are present. These patients are said to have atypical CML which is classified as a separate disease by the World Health Organisation.

More information on atypical CML can be found in the Atypical CML booklet on the Leukaemia Care website at