A low monocyte count, called monocytopenia, can mean a person has an underlying infection or bone marrow problem. Monocytopenia can be caused by any conditions that lower the body's white blood cell count.
Monocytes are a type of white blood cell. They make up anywhere from one to 10 percent of all white blood cells in the human body. Monocytes are produced in bone marrow before they are dispersed into the bloodstream. Once released, they work with other white blood cells to fend off infection, eliminate dead tissue, eradicate cancer cells and boost immunity against harmful pathogens. They make their way to bone marrow tissue and other organs like the lungs, spleen and kidneys, where they mature into adult cells called macrophages. Macrophages, like monocytes, protect the body against disease and infection. Sometimes, genetic conditions affect the function of macrophages. This results in lipid accumulation inside cells. Genetic conditions in this category include Niemann-Pick disease and Gaucher disease.
What a Low Monocyte Count Means
While some inherent conditions result in a low monocyte count, monocytopenia can also arise from acquired conditions. At any given time, the bloodstream should have 200 to 600 monocytes per microliter of blood. Repeated blood tests that show a number below this level can mean that a person has an infection or certain types of cancer, including hairy-cell leukemia. Individuals who are undergoing chemotherapy treatment can also have low monocyte levels. Occasionally, test results will show that a person has no monocytes at all. This condition is called MonoMAC syndrome, and it means that that a person's blood does not have or produce monocytes.
MonoMAC is a genetic defect that affects bone marrow. It weakens the body's immunity, so people with MonoMAC tend to get more frequent infections than those without it. Additionally, they are more likely to become infected by microorganisms that are usually harmless to humans. The symptoms of MonoMAC depend on the underlying microorganism causing the infection. Doctors usually diagnose MonoMAC through blood test results that show the absence of monocytes. Treatment includes a bone marrow transplant and sometimes antibiotics to get rid of the infection.
Other White Blood Cells
Monocytes are one of five types of white blood cell analyzed in a white blood cell (WBC) count. Eosinophils, basophils, neutrophils and lymphocytes are the other four kinds of white blood cells. As part of a blood test, doctors will look at the ratio of all white blood cells to check for outlying numbers. Sometimes, more than one type of white blood cell can be higher or lower than average. Having multiple types of high or low white blood cells can help doctors make a diagnosis. If an infection is causing the low monocyte count, a physician can sometimes determine whether it is due to a virus or bacteria based on the WBC results. The white blood cell test can also show if there are immature cells in the blood or reveal structural abnormalities in any type of white blood cell.
Because monocytopenia does not cause signs or symptoms, people might only discover that they have the condition during a routine blood test. A doctor might order further tests, including a bone marrow biopsy, to find out what is causing the low monocyte count.