What is the role of blood count and peripheral smear in the workup of chronic myelogenous leukemia (CML)?

Updated: May 23, 2021
  • Author: Emmanuel C Besa, MD; Chief Editor: Sara J Grethlein, MD, FACP  more...
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Answer

In CML, the increase in mature granulocytes and normal lymphocyte counts (low percentage due to dilution in the differential count) results in a total WBC count of 20,000-60,000 cells/μL. A mild increase in basophils and eosinophils is present and becomes more prominent during the transition to acute leukemia.

These mature neutrophils, or granulocytes, have decreased apoptosis (programmed cell death), resulting in accumulation of long-lived cells with low or absent enzymes, such as alkaline phosphatase (ALP). Consequently, the leukocyte alkaline phosphatase stains very low to absent in most cells, resulting in a low score.

The peripheral blood smear in patients with CML shows a typical leukoerythroblastic blood picture, with circulating immature cells from the bone marrow (see the image below).

Chronic myelogenous leukemia. Blood film at 400X m Chronic myelogenous leukemia. Blood film at 400X magnification demonstrates leukocytosis with the presence of precursor cells of the myeloid lineage. In addition, basophilia, eosinophilia, and thrombocytosis can be seen. Courtesy of U. Woermann, MD, Division of Instructional Media, Institute for Medical Education, University of Bern, Switzerland.

The transitional or accelerated phase of CML is characterized by poor control of blood counts with myelosuppressive medication, the appearance of peripheral blast cells (≥15%), promyelocytes (≥30%), basophils (≥20%), and reduction in platelet counts to less than 100,000 cells/μL unrelated to therapy. Promyelocytes and basophils are shown in the images below.

Chronic myelogenous leukemia. Blood film at 1000X Chronic myelogenous leukemia. Blood film at 1000X magnification shows a promyelocyte, an eosinophil, and 3 basophils. Courtesy of U. Woermann, MD, Division of Instructional Media, Institute for Medical Education, University of Bern, Switzerland.
Chronic myelogenous leukemia. Blood film at 1000X Chronic myelogenous leukemia. Blood film at 1000X magnification demonstrates the whole granulocytic lineage, including an eosinophil and a basophil. Courtesy of U. Woermann, MD, Division of Instructional Media, Institute for Medical Education, University of Bern, Switzerland.

Signs of transformation or accelerated phase in patients with CML are poor control of blood counts with myelosuppression or interferon, increasing blast cells in peripheral blood with basophilia and thrombocytopenia not related to therapy, new cytogenetic abnormalities, and increasing splenomegaly and myelofibrosis.

In approximately two thirds of cases, the blasts are myeloid. However, in the remaining one third of patients, the blasts exhibit a lymphoid phenotype, further evidence of the stem cell nature of the original disease. Additional chromosomal abnormalities are usually found at the time of blast crisis, including additional Ph1 chromosomes or other translocations.

Early myeloid cells such as myeloblasts, myelocytes, metamyelocytes, and nucleated red blood cells are commonly present in the blood smear, mimicking the findings in the bone marrow. The presence of the different midstage progenitor cells differentiates CML from the acute myelogenous leukemias, in which a leukemic gap (maturation arrest) or hiatus exists that shows absence of these cells.

A mild to moderate anemia is very common at diagnosis and is usually normochromic and normocytic. The platelet counts at diagnosis can be low, normal, or even increased in some patients (>1 million in some).


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