Leukemoid reaction and chronic myeloid leukemia (CML) can both cause a high white blood cell count. However, they typically increase the levels of different types of white blood cells.

CML and leukemoid reaction affect different types of white blood cells. CML increases immature forms of granulocytes, and a leukemoid reaction increases mature and immature neutrophils.

Granulocytes provide a broad immune defense for the body. In comparison, neutrophils are a type of granulocyte that focuses on fighting infections and healing wounds.

Leukemoid reactions can have several causes, but CML is the result of blood cancer. The underlying cause affects how doctors treat each condition.

Keep reading to learn more about the similarities and differences between leukemoid reaction and CML.

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A leukemoid reaction involves having an elevated white blood cell count, ranging from 50,000–100,000 white blood cells per cubic millimeter. This count is due to an increased number of neutrophils, which help the body fight bacterial, viral, and fungal infections. They also support the body to heal wounds.

Causes

The causes of the high white blood cell count can include:

The symptoms will vary depending on the cause.

Diagnosis

Older research reports that the condition is rare and requires careful diagnosis. This process involves ruling out blood cancers, such as CML.

A thorough check of a person’s medical history may highlight a factor that increases their risk of leukemoid reaction, such as a recent infection. Further testing is sometimes necessary to differentiate a leukemoid reaction from CML. Such tests may include:

Treatment

Treatment of leukemoid reaction involves addressing the underlying cause and reducing the person’s white blood cell count. For instance, a doctor may prescribe antibiotics to someone with a leukemoid reaction that is due to a bacterial infection. Antibiotics help the body kill bacteria and fight infections.

CML is a blood cancer in which having excess stem cells in the bone marrow increases white blood cell counts to abnormal levels. Leukemoid reaction also involves a high white blood cell count, which is a similarity between the two conditions.

Leukemoid reaction and CML both elevate white blood cells levels, but each condition affects a different type of white blood cell. Leukemoid reactions cause an increase in neutrophils, whereas CML increases granulocyte levels.

Additionally, the excess neutrophils in a leukemoid reaction are mostly mature cells that can function normally. The granulocytes in CML are abnormal and unable to function properly.

Another difference between these conditions is their symptoms. A person with a leukemoid reaction does not typically experience the symptoms of leukemia. However, someone with CML can experience these symptoms, which include:

A further difference between these conditions is their causes.

Leukemoid reactions stem from medical conditions outside of the bone marrow and are not necessarily the result of cancer. However, CML is the result of cancer that starts in the bone marrow.

There are also several differences in how doctors interpret laboratory results for each condition.

According to older research, the primary difference appears in the peripheral blood test. A leukemoid reaction may be present if the test shows mature neutrophils with a marked left shift. A left shift indicates the presence of immature neutrophils in the blood.

A diagnosis of CML may cause the peripheral blood test to show increased immature basophils and eosinophils.

Leukemoid reactions and CML are serious conditions that always require medical attention. People who experience signs or symptoms of either condition should contact a doctor right away.

Doctors use diagnostic tests to identify the cause of the symptoms. In some cases, they will prescribe treatments for the underlying cause, which could be cancer.

A 2020 clinical trial looked at the causes and outcomes of leukemoid reactions in a Brazilian hospital between 2016 and 2018. It found that infections were the most common cause, but many cases involved a cancerous tumor. The authors suggested that both causes lead to a high death rate.

The American Cancer Society reports that, in a large trial, 90% of people with CML who received imatinib as a treatment survived for at least 5 years after their initial diagnosis.

Leukemoid reaction and CML both lead to increases in the number of white blood cells in the blood. However, they affect different white blood cell types. Leukemoid reactions cause an increase in neutrophils, whereas CML increases granulocytes. These cells have similar but different functions.

A key distinction is that CML is the result of blood cancer, which typically begins in the bone marrow. However, leukemoid reactions can have several causes, such as infections. These differing causes produce different symptoms and require different treatment approaches by doctors.