Routine complete blood cell counts, which include a white blood cell count, are often ordered during annual physical exams, according to WebMD. A white blood cell count should be conducted more regularly in patients with a medical condition that affects white blood cells, notes the University of Rochester Medical Center.
Some conditions or medical treatments, such as HIV or chemotherapy, lower white blood cell counts, reports the University of Rochester Medical Center. In these at-risk patients, white blood cell counts are measured more often, as determined by a doctor. Because white blood cells are responsible for fighting infection, a low white blood cell count can make a patient more susceptible to infection.