Why is there more than one test for white blood cell count?

A standard blood count calculates the number of white blood cells (WBCs) per microliter of blood. This is also called the total leukocyte count, or TLC. A healthy person has between 4,500 and 11,000 of them. A second test, called a differential count, or "diff," breaks down the count into five different types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Healthy blood is made up of approximately 60 percent neutrophils, 30 percent lymphocytes, 5 percent monocytes, 4 percent eosinophils, and 1 percent basophils. Knowing which types of white blood cells have been most affected helps the doctor evaluate a patient's risk of becoming immune suppressed.

One way doctors measure this risk is with a test called the absolute neutrophil count, or ANC, which calculates the percent of neutrophils in the total white blood count. The risk of infection increases as the ANC falls; if someone's ANC falls below 1,000, he has a moderate risk of infection, while an ANC under 500 carries a high risk of infection.