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Overview of Leukemia


Ashkan Emadi

, MD, PhD, University of Maryland;

Jennie York Law

, MD, University of Maryland, School of Medicine

Last full review/revision May 2020| Content last modified May 2020
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Leukemia is a malignant condition involving the excess production of immature or abnormal leukocytes, which eventually suppresses the production of normal blood cells and results in symptoms related to cytopenias.

Malignant transformation usually occurs at the pluripotent stem cell level, although it sometimes involves a committed stem cell with more limited capacity for self-renewal. Abnormal proliferation, clonal expansion, aberrant differentiation, and diminished apoptosis (programmed cell death) lead to replacement of normal blood elements with malignant cells.

The American Cancer Society estimates that in the United States in 2020 there will be about 60,530 new cases of leukemia (of all types) in adults and children, and about 23,100 deaths.

Classification of Leukemia

The current approach to classifying leukemia is based on the 2016 World Health Organization (WHO) system (classification for hematopoietic neoplasms). The WHO classification is based on a combination of clinical, morphologic, immunophenotypic, and genetic features. Other less commonly used classification systems include the French-American-British (FAB) system, which is based on the morphology of the abnormal leukocytes.

Leukemias are commonly also categorized as

  • Acute or chronic: Based on the percentage of blasts or leukemia cells in bone marrow or blood

  • Myeloid or lymphoid: Based on the predominant lineage of the malignant cells

The four most common leukemias and their distinguishing features are summarized in the table Findings at Diagnosis in the Most Common Leukemias.

For 2020, the American Cancer Society estimated the distribution of new US cases by leukemia type as follows:


Findings at Diagnosis in the Most Common Leukemias


Peak age of incidence


Any age

Middle and old age


White blood cell count

High in 50%

Normal or low in 50%

High in 60%

Normal or low in 40%

High in 98%

Normal or low in 2%

High in 100%

Differential white blood cell count

Many lymphoblasts

Many myeloblasts

Small lymphocytes

Entire myeloid series


Severe in > 90%

Severe in > 90%

Mild in about 50%

Mild in 80%


Low in > 80%

Low in > 90%

Low in 20 to 30%

High in 60%

Low in 10%







In 60%

In 50%

Usual and moderate

Usual and severe

Other features

Without prophylaxis, central nervous system commonly involved

Central nervous system rarely involved

Sometimes Auer rods in myeloblasts

Occasionally hemolytic anemia and hypogammaglobulinemia

Low leukocyte alkaline phosphatase level

Philadelphia chromosome–positive in > 90%

Acute leukemias

Acute leukemias consist of predominantly immature, poorly differentiated cells (usually blast forms). Acute leukemias are divided into acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).

Chronic leukemias

Chronic leukemias have more mature cells than do acute leukemias. They usually manifest as leukocytosis with or without cytopenias in an otherwise asymptomatic person. Findings and management differ significantly between chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML).

Myelodysplastic syndromes

Myelodysplastic syndromes are a group of clonal hematopoietic stem cell disorders unified by the presence of distinct mutations of hematopoietic stem cells. They involve progressive bone marrow failure but with an insufficient proportion of blast cells (< 20%) for making a definite diagnosis of acute myeloid leukemia; 40 to 60% of cases evolve into acute myeloid leukemia.

Leukemoid reaction

A leukemoid reaction is a neutrophil count > 50,000/mcL (> 50 × 109/L) not caused by malignant transformation of a hematopoietic stem cell. It can result from a variety of causes, particularly other cancers or systemic infection. Usually the cause is apparent, but apparent benign neutrophilia can be mimicked by chronic neutrophilic leukemia or chronic myeloid leukemia.

Risk Factors for Leukemia

Risk of developing leukemia is increased in patients with

  • History of exposure to ionizing radiation (eg, post–atom bomb in Nagasaki and Hiroshima) or to chemicals (eg, benzene, some pesticides, polyaromatic hydrocarbons in tobacco smoke); exposure can lead to acute leukemias

  • Prior treatment with certain antineoplastic drugs, including alkylating agents, topoisomerase II inhibitors, hydroxyurea, and maintenance lenalidomide after autologous stem cell transplantation with melphalan-containing conditioning regimens for multiple myeloma; can lead to a type of acute myeloid leukemia called t-AML or therapy-related AML

  • Infection with a virus (eg, human T lymphotropic virus 1 and 2, Epstein Barr virus) can rarely cause certain forms of ALL; this is seen mainly in regions where such infections are common, such as Asia and Africa

  • History of antecedent hematologic disorders, including myelodysplastic syndromes and myeloproliferative neoplasms , which can lead to AML

  • Preexisting genetic conditions (eg, Fanconi anemia, Bloom syndrome, ataxia-telangiectasia, Down syndrome, xeroderma pigmentosum, Li-Fraumeni syndrome) predispose to acute myeloid leukemia and acute lymphoblastic leukemia.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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