What's measured in a blood cell count?

If you're undergoing certain cancer treatments that could cause low blood cell counts, your doctor will likely monitor your blood cell counts regularly using a test called a complete blood count with differential (CBC w/diff). Low blood cell counts are detected by examining a blood sample taken from a vein in your arm.

When checking your blood cell count, your doctor is looking at the numbers and types of the following:

• White blood cells. White blood cells help your body fight infection. The CBC w/diff test measures the levels of five types of white blood cells in your blood. Your doctor will pay close attention to the levels of neutrophils, which are particularly helpful for fighting infections caused by bacteria. A low neutrophil count (neutropenia) leaves your body more open to infection. And if an infection does develop, your body may be unable to fight it off.
• Red blood cells. Red blood cells carry oxygen throughout your body. Your red blood cells' ability to carry oxygen is measured by the amount of hemoglobin in your blood. If your level of hemoglobin is low, you're anemic and your body works much harder to supply oxygen to your tissues. This can make you feel fatigued and short of breath.
• Platelets. Platelets help your blood to clot. A low platelet count (thrombocytopenia) means your body can't stop itself from bleeding.