What Is a Monocyte Blood Test?

A monocyte blood test measures the number of monocytes, a type of white blood cell, in the blood. Doctors use the test to evaluate the function of the immune system and the bone marrow. It is also used to look for certain diseases, such as monocytic leukemia.

A monocyte count is typically part of a more comprehensive blood test known as a complete blood count with differential. This test measures the level of red blood cells and specific types of white blood cells in a volume of blood. The test result usually is reported as a percentage of the total number of white blood cells. Normal levels are between 2 percent and 10 percent.

Monocytes are produced in the bone marrow and then released into the bloodstream. They help other white blood cells remove dead or damaged cells, fight infections and destroy cancer. A high number of monocytes in the blood is called monocytosis and often indicates a chronic infection, a blood disorder or an autoimmune disorder, such as rheumatoid arthritis. Monocytosis is sometimes indicative of cancer or leukemia.

A low number of monocytes in the blood is called monocytopenia. This condition is associated most often with bone marrow suppression caused by chemotherapy treatments or certain bacterial infections.