What is a complete blood count?

A complete blood count is a test that provides important information about the types and numbers of cells in the blood, including red blood cells, white blood cells and platelets, according to WebMD. A complete blood count helps doctors evaluate symptoms and diagnose conditions.

To perform a complete blood count, a health professional takes a blood sample from the patient's vein and sends it to a lab for analysis, says Healthline. If the complete blood count shows unusual levels of certain types of cells, doctors may order another blood test or other tests to further investigate or confirm a diagnosis, as blood cell counts that are too high or too low may be a sign of certain medical issues.