Blood test that determines number of leukocytes; increase indicates infection or leukemia; decrease may be caused by some diseases, radiation therapy, or chemotherapy.

• A. Hematocrit
• B. Platelet count
• C. White blood cell count (WBC)
• D. Red blood cell count

Answer: (C)

Measuring leukocytes is done with a white blood cell count. This test directly reflects the number of immune cells circulating in the blood. An increased white blood cell count, or leukocytosis, often points to infection or inflammation and can be seen in leukemia. A decreased count, leukopenia, may result from certain diseases or from treatments like radiation therapy or chemotherapy that suppress bone marrow.

The other values measure different blood components: hematocrit assesses the proportion of blood volume made up by red blood cells, platelet count tracks the cells involved in clotting, and red blood cell count counts the cells responsible for oxygen transport. None of these specifically measures leukocytes, which is why the white blood cell count is the correct choice.