What Is Leukocytosis?

Medically Reviewed by Dan Brennan, MD on June 01, 2021

White blood cells, called leukocytes, are important for making sure your body stays healthy. They protect it from alien substances and fight infection. A higher-than-normal number of white blood cells is called leukocytosis. Leukocytes usually increase in number when they are doing their job, but there are some other conditions that can cause an increase.

What qualifies as a normal white blood count isn't the same for everyone. A normal count for adults is from 4,500 to 11,000 per microliter. Newborns have a much higher number — as high as 38,000. This count declines as the child grows, eventually falling to adult levels. Pregnant women have slightly higher counts. ‌

White Blood Cells

Although leukocytes are very important, they make up only about 1% of your blood. Like other blood cells, they are made in the bone marrow. They are being created all the time because most have a very short lifespan. Some live for less than a day. 

There are five major types of white blood cells:‌

  • neutrophils
  • eosinophils
  • monocytes
  • lymphocytes‌
  • basophils ‌

When your body is under attack, the white blood cells rush to the problem area. Different types of white blood cells have different roles in fighting invaders: Neutrophils fight bacteria, eosinophils fight parasites, monocytes fight foreign organisms and also deal with cell waste, and lymphocytes make up the immune system. The role of basophils isn't clear, but they are also involved in the immune response. 

How Is Leukocytosis Diagnosed?

Most of the time, doctors use a complete blood count (CBC) to check for leukocytosis. A CBC can be part of a routine physical, or your doctor might use it to help diagnose a specific illness. Another test, called a white blood cell differential or "diff," is sometimes done at the same time. It supplies counts for the five main types of white blood cells.

You can have a high count of all five types of cells, a few types, or just one type — usually lymphocytes or neutrophils. A high count of these cells is sometimes associated with blood cancers. 

Lymphocytic Leukocytosis

Lymphocytes are an important part of the body's defense system. They protect against bacteria, viruses, parasites, and fungal growth. A high count of these cells is called lymphocytic leukocytosis. Lymphocytes come in three types: B cells, T cells, and natural killer cells. By determining which type is elevated, doctors can find out the exact cause of the high count.

‌The most common cause of lymphocytic leukocytosis is infection with a virus. Other causes include:‌

  • tuberculosis, a bacterial infection of the lungs
  • Graves’ disease, a disorder of the thyroid gland
  • Crohn's disease, an inflammatory bowel disease

Two blood cancers, lymphoma and lymphocytic leukemia, are associated with high numbers of lymphocytes in the bloodstream. 

Neutrophilic Leukocytosis

Neutrophils, like lymphocytes, protect against bacteria, viruses, parasites, and fungal growth. They also play a role in healing injuries. Myeloid leukemias can cause high neutrophil counts. If doctors find a high number of immature neutrophils in the blood, they may suspect leukemia. Doctors usually diagnose leukemia by analyzing a bone marrow sample.‌‌

A serious condition can result if a person has leukemia and the number of immature neutrophils in the blood goes too high. This rare condition is called hyperviscosity syndrome, and it can happen when the blood becomes too thick. It can cause a stroke or breathing problems that could lead to death. Doctors treat this syndrome by adding fluid to the blood and using drugs to reduce the neutrophils in the blood.

Non-Malignant Causes of Leukocytosis

Disorders of the white blood cells can be malignant or non-malignant. The main non-malignant cause of leukocytosis is infection. A high white blood count most often signals an infection in the body, especially when there are also other symptoms. ‌

Other conditions that can cause leukocytosis include:

  • lifestyle factors such as obesity and smoking
  • chronic conditions such as rheumatoid arthritis or inflammatory bowel disease
  • genetic conditions such as Down syndrome

Malignant Causes of Leukocytosis

‌Leukocytosis can also signal malignant disorders, especially leukemias or other blood cancers. Doctors look at several factors when considering this diagnosis.‌

Symptoms. Fever, night sweats, and weight loss are three symptoms that suggest a malignant cause of leukocytosis. Doctors also look for evidence of bleeding or easy bruising. Fatigue may be present.

Physical examination. Doctors check for swollen lymph nodes, an enlarged liver, and an enlarged spleen. They also look for petechiae, pinpoint spots caused by bleeding under the skin.‌

Lab results. Blood cancers are the typical cause of white blood counts over 30,000 per microliter. Some of the cells may be immature. Other counts, such as red blood cells and platelets, may be low.

Show Sources


American Family Physician: "Evaluation of Patients With Leukocytosis."

Cleveland Clinic: "High White Blood Cell Count."

Memorial Sloan Kettering Cancer Center: "Facts About Blood and Blood Cells." 

Merck Manual Consumer Version: "High White Blood Cell Count," "Lymphocytic Leukocytosis," "Neutrophilic Leukocytosis," "Overview of White Blood Cell Disorders."

Radiopaedia: "Hepatosplenomegaly."

University of Rochester Medical Center: "What Are White Blood Cells?"

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