This is the single most important test for understanding your body's ability to fight bacterial infections. It measures the absolute number of neutrophils, your immune system's 'first responders.' This test is critical for patients undergoing chemotherapy, as a low count (neutropenia) signals a high risk of infection. It is the definitive test for assessing your body's immediate infection-fighting reserves. This simple blood test provides the clear, actionable number your doctor needs to keep you safe.