In most cases CML is a biphasic or triphasic disease. The initial
chronic phase may last several years without change, but ultimately it
either changes abruptly to an acute phase or, more commonly, evolves
slowly into a phase of "acceleration," which later progresses to the
acute phase. The acute phase is usually defined by the observation of
>30% blasts or blasts plus promyelocytes in the blood or marrow,57 but
criteria for defining the accelerated phase are imprecise.58 A
reasonable classification has been proposed by the International Bone
Marrow Transplant Registry (Table 73-2).

Table 73-2. Criteria Established by the International Bone Marrow
Transplant Registry for Classifying the Phases of CML

1).   Chronic phase

No significant symptoms (after treatment);

None of the features of accelerated phase or blastic phase.

2).  Accelerated phase

WBC count difficult to control with conventional use of busulfan or

urea in terms of doses required or shortening of intervals between
courses Rapid doubling of WBC count t<5 days) =-:10% blasts in blood or
marrow 2:20% blasts plus promyelocytes in blood or marrow a:20%
basophils plus eosinophils in blood Anemia or thrombocytopenia
unresponsive to busulfan or hydroxyurea Persisent thrombocytosis

Additional chromosome changes (evolving new clone) Increasing
splenomegaly Development of chloromas or myelofibrosis

Blastic phase

2:30% blasts plus promyelocytes in the blood or bone marrow

(Data from International Bone Marrow Transplant Registry.)