A low white blood cell count, called leukopenia, may be caused by bone marrow problems, autoimmune disorders such as lupus, diseases involving the liver or spleen, certain viral illnesses or severe bacterial infections, states MedlinePlus. Radiation treatment and medications used for cancer treatment may also decrease white blood cell count.
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.