Understanding Complete Blood Count
It includes information about red blood cells, white blood cells, platelets, hemoglobin, and hematocrit. A CBC with differential adds information about the different kinds of white blood cells.
How it is assessed or used in care
A clinician may use a CBC to investigate fatigue, infection, bleeding, or other concerns, or to monitor a condition or treatment. Abnormal results can have many causes and do not identify one diagnosis on their own.
What to discuss with your clinician
A blood sample is usually taken from a vein. A CBC alone commonly does not require fasting, but other tests drawn at the same time may. Ask which results need follow-up and how they compare with previous tests and the laboratory’s reference ranges.
Medical references
Sources accessed November 13, 2026.
This information is for general education, not a diagnosis or an individualized treatment plan. Discuss symptoms, test results, and treatment decisions with a qualified clinician. For a medical emergency, call 911.