A leukemoid reaction is a very large short-term increase in your white blood cell count that can be mistaken for leukemia. Some of the potential causes include:

  • infections
  • solid cancers
  • drug side effects

Leukemia is a group of cancers that develop in cells that produce blood cells in your bone marrow. People with leukemia often have extremely elevated white blood cell counts.

Having a leukemoid reaction doesn’t mean you have cancer, but it can be a sign of a potentially serious medical condition that needs treatment.

Read on to learn more about leukemoid reactions, including what they are and what causes them.

A leukemoid reaction is defined as a white blood cell count above 50,000 cells per microliter (µL) of blood. A microliter is one-millionth of a liter.

For reference, the Leukemia & Lymphoma Society lists normal white blood count ranges as:

Demographic White blood cells per microliter (µL)
Women (nonpregnant)4,500–11,000

Bloodwork of a person with a leukemoid reaction usually shows an increase in mature neutrophils and immature granulocytes. These white blood cells play an important role in your immune system’s first-line defense.

Neutrophils are the most common white blood cells in your bloodstream. They act as first responders to destroy bacteria and other foreign invaders.

Granulocytes are released during infection and inflammatory conditions. They help destroy foreign invaders and signal for other white blood cells to attack them, too.

What’s the difference between a leukemoid reaction and leukemia?

A leukemoid reaction is an elevated white blood cell count caused by an underlying condition that isn’t a blood cancer. Leukemia is a group of blood cancers that form in the cells that create blood cells.

A blood smear can help doctors or healthcare professionals understand which condition you have. During this test, your blood cells are examined microscopically to see how many white blood cells you have and whether they look abnormal.

People with a type of leukemia called chronic myeloid leukemia often show an increase in abnormal granulocytes, whereas a leukemoid reaction is more likely to be characterized by an increase in mature neutrophils.

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Doctors can further classify a leukemoid reaction based on the elevated type of white blood cells.

  • Granulocytic reaction: A granulocytic reaction is the most common type. It’s usually characterized by an increase in the number of neutrophils. It can be a sign of infection, some cancers, and other conditions.
  • Lymphocytic reaction: A lymphocytic reaction is characterized by an increase in a group of white blood cells called lymphocytes. A lymphocytic reaction is often a sign of an immune reaction against cancer cells.
  • Monocytic reaction: A monocytic reaction is an increase in white blood cells called monocytes. These cells multiply in response to infection or injury.

A leukemoid reaction is caused by an underlying condition. Symptoms vary depending on which underlying condition you have. Here’s a general look at how symptoms of some common causes compare to general symptoms of leukemia.

LeukemiaInfectionCancerAlcohol hepatitis
fever or chillsfever or chillsspecific organ symptomsfever
fatiguefatiguefatiguenausea or vomiting
weaknesscoughskin changesdry mouth
frequent infectionssore throatchange in bowel habitsabdominal pain
unintentional weight lossdiarrhea or vomitingunintentional weight lossunintentional weight loss
easy bleeding or bruisingabdominal painpoor wound healingeasy bleeding or bruising
swollen lymph nodesshortness of breathlump under skinjaundice

A leukemoid reaction can be because of a variety of conditions that cause inflammation or stress in your body. Conditions that have been linked to leukemoid reactions include:

In a 2020 study published in the International Journal of Laboratory Hematology, researchers analyzed the underlying cause of 267 cases of white blood cell counts above 50,000 cells per µL of blood in Brazilian adults over a 2-year period. The researchers found that 60% of cases were caused by blood cancers. Of the 40% that made up leukemoid reactions:

  • 56% were caused by infection
  • 16% were caused by other solid cancers
  • 28% were caused by other conditions

Small studies have reported leukemoid reactions in 1 to 4% of non-blood cancers.

People with Down syndrome frequently have elevated white blood cell counts within the first few months of life that usually resolve themselves.

Can COVID-19 cause a leukemoid reaction?

COVID-19 can affect many parts of your body, including causing changes in your white blood cell count. Low white blood cell counts have been reported more often than high blood cell counts in people with COVID-19, but several case studies report present people with leukemoid reactions.

A 2021 case study reported a 36-year-old woman with severe COVID-19 who developed a leukemoid reaction presumed to be caused by COVID-19. The woman passed away 14 days after hospital admission.

Another 2021 case study reported a 64-year-old woman with severe COVID-19 diagnosed with a leukemoid reaction. She died 13 days after hospital admission.

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A leukemoid reaction is diagnosed when you have a white blood cell count above 50,000 cells per µL of blood and don’t have blood cancer. Doctors can draw a sample of your blood and perform a complete blood cell count to measure your white blood cell count.

Doctors can usually easily differentiate a leukemoid reaction from leukemia by performing a peripheral blood smear. A peripheral blood smear is a test where doctors look at your blood microscopically.

A doctor may run a variety of other tests to rule out other conditions or find the underlying cause. Tests may include:

  • other blood or urine tests
  • physical exam
  • imaging
  • tissue biopsies
  • genetic testing

A leukemoid reaction is treated by targeting the underlying condition that’s causing it. For example, a bacterial infection, such as tuberculosis, is treated with antibiotics.

Underlying cancer may be treated with therapies such as:

Alcohol hepatitis may be treated with:

  • ceasing alcohol consumption
  • vitamin and nutrient supplements
  • liver transplant
  • medications
  • counseling

The outlook for someone with a leukemoid reaction depends on the underlying cause. Blood cells usually return to normal levels when the underlying condition is treated.

A highly elevated blood cell count may be a sign that an infection is advanced. In the 2020 study published in the International Journal of Laboratory Hematology, researchers found that people with leukemoid reactions associated with infections and cancers had poor outlooks.

A leukemoid reaction is a highly elevated white blood cell count caused by an underlying disease that isn’t a blood cancer. Many different conditions can cause a leukemoid reaction such as infections, alcohol hepatitis, and other cancers.

A doctor can help you figure out why your white blood cell count is elevated by running blood and other tests. If the underlying cause is curable, your blood cell count will likely return to normal once it’s treated.