What Causes a Low White Blood Cell Count?

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A low white blood cell count can be related to a number of causes, including viral infections, cancer and diminished immune system function, according to MayoClinic. Detailed blood screening is the first step necessary to determine the specific cause.

A low white blood cell count is called leukopenia. Leukopenia is best defined as a decrease in the number of white blood cells, called leukocytes, in the blood. The test that determines blood cell count is the white blood cell (WBC) test, which is often included as one part of the complete blood count (CBC) test.

Causes of low WBC include HIV, bone marrow damage or disorders, viral infections, leukemia and other illnesses. When diagnosing low WBC causes, one must take into account other factors, including medical history and current symptoms.

Taking certain medications, including antibiotics, anticonvulsants, captopril, diuretics or sulfonamides, may lower white blood cell count, according to MedlinePlus. Ticlopidine, quinidine, histamine-2 blockers, clozapine and anti-thyroid medications are also capable of decreasing the number of white blood cells. A low white blood cell count is defined as less than 4,500 white blood cells per microliter of blood, or less than 1,700 neutrophils per microliter of blood. Neutrophils are a specific type of white blood cell that help fight infections. A normal white blood cell count ranges from 4,500 to 10,000 white blood cells per microliter.

Doctors usually use a variety of tests to determine the underlying cause of a low white blood cell count, states Mayo Clinic. Because patients with a persistently low white blood cell count are at increased risk for infection, special precautions are necessary to keep them from getting sick. Some recommendations include washing hands frequently, avoiding individuals who are ill and wearing a face mask in public places.