The white blood cell (WBC) count measures the total number of all leukocytes in the blood: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The WBC is reported as a total count and as a differential (the proportion of each cell type), which provides critical information about the nature of any abnormality.

Elevated WBC (leukocytosis) most commonly indicates bacterial infection, but also occurs with inflammation, tissue injury, steroid therapy, and haematological malignancy. Low WBC (leukopenia) increases infection risk and occurs with viral infections, autoimmune conditions, chemotherapy, and bone marrow failure. The WBC is automatically reported as part of every full blood count.