Q. I’d like to know how to differentiate between acute leukemia and blast crisis of CML.

A. Great question! To diagnose blast crisis (or blast phase) in chronic myeloid leukemia, you need to have one of the following:

  1. More than 20% blasts in the blood or bone marrow
  2. An extramedullary focus of blasts (e.g., in the skin, lymph nodes, or spleen)
  3. A significant focus of blasts in the bone marrow section

Most of the time, blast crisis presents as a straight-up acute leukemia (more than 20% blasts in the blood or bone marrow). So how would you tell blast crisis apart from a de novo acute leukemia?

Well, you could try to find previous material from the patient – an older bone marrow or blood – and see if there is evidence of CML. Or you could look for the BCR-ABL translocation. This translocation (also called the Philadelphia chromosome) is present in all cases of CML (check it out: ALL cases!) and is only present in a very, very small number of acute leukemias (rare cases of acute lymphoblastic leukemia have the Philadelphia chromosome – if you look closely, though, the breakpoint is different).

But assuming you can’t do those things, the best thing to do is look at the remainder of the cells (the non-blasts). In blast crisis, the remainder of the cells would reflect the underlying CML (a case of which is shown above): you’d see a large percentage of neutrophilic cells (with a left shift) and a basophilia. In a regular old acute leukemia, the remainder of the cells would show a normal differential (more neutrophils than lymphocytes, and a very small number of basophils), and no left shift.