How does a CBC test work?
During a CBC test, a special computer counts and looks at the size of red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). If your provider orders a CBC with differential, it also counts the different types of white blood cells and immature cells in your sample. If there’s anything unclear about your results, a pathologist will look at your sample under a microscope to verify the computer’s results.
Red blood cell counts
In a CBC for red blood cells, your providers look at:
- Red blood count (RBC). This is the number of red blood cells in a certain amount of blood, usually a microliter (mcL). For reference, one drop of blood is about 40 mcL to 50 mcL.
- Hemoglobin (Hb). This is the amount of hemoglobin in your blood.
- Hematocrit (Hct). This is the percentage of your blood made up of red blood cells.
You might also see indices, which describe the size of your red blood cells and hemoglobin concentrations. These include:
- Mean corpuscular volume (MCV). This is the average size of your red blood cells.
- Mean corpuscular hemoglobin (MCH). This is the average concentration (weight) of hemoglobin in each red blood cell.
- Mean corpuscular hemoglobin concentration (MCHC). This is the average concentration (weight) of hemoglobin in a certain amount of blood.
- Red cell distribution width (RDW). This is the degree of difference in red blood cell size. In other words, if the number is low, all of your blood cells are close to the same size. If it’s high, there’s a big difference between your largest and smallest red blood cells.
Platelet count
The results of your platelet count on a CBC include:
- Platelet count. The total number of platelets in your blood sample.
- Mean platelet volume (MPV). MPV is your average platelet size.
White blood cell counts and differentials
Your provider can order a CBC with just total white blood cell counts or with a differential. A differential counts each different type of white blood cell and immature red blood cells. These might be shown as an absolute number of cells (written as “abs”) or a percentage (%) of all of your white blood cells.
You might see on your CBC report:
- White blood cell count (WBC). This is a count of your total white blood cells (of all types).
- Neutrophils. These are the first responders of white blood cells. They fight bacterial and fungal infections.
- Monocytes. Monocytes are white blood cells that clean up cell debris during an infection. They also help alert other white blood cells to harmful intruders in your body, like bacteria and viruses.
- Lymphocytes. Lymphocytes are a type of white blood cell that fights viral infections and helps your immune system remember previous infections. This prepares your body so it can better fight off the same kind of infection in the future.
- Eosinophils. These white blood cells fight parasitic infections and cause allergic reactions.
- Basophils. Basophils are a type of white blood cell that releases histamine during allergic reactions and heparin, which prevents blood from clotting.
- Immature granulocytes. Immature granulocytes are neutrophils, eosinophils or basophils that haven’t fully developed yet.
- Nucleated red blood cells (nRBC). NRBCs are immature red blood cells. They can be listed as the total number (absolute nRBC) or a percentage (the number of red blood cells per 100 white blood cells).
How do I prepare for a CBC test?
You don’t need to fast or follow special instructions to prepare for a CBC. You can eat, drink and take your medications as you normally would. But if your provider is taking samples for other blood tests at the same time, you may have special instructions for those tests.
What can I expect during a CBC test?
To get a blood sample, your provider cleans your arm and inserts a needle. The needle may sting or pinch a little. In infants, providers usually insert the needle into the baby’s heel.
Through the needle, your provider removes a sample of your blood and collects it in a tube. Sometimes, your provider takes more than one tube of blood.
After drawing blood, your provider removes the needle and places a bandage on your arm. Your provider sends the blood to a lab for testing.
What can I expect after a CBC test?
After a blood draw for a CBC test, you’ll have some gauze and a bandage on your arm, secured with tape. Your arm may be a little sore for a few hours. You may develop a small bruise where your provider inserted the needle.
Are there risks or side effects to a CBC test?
A CBC is a safe, common test. There’s a small risk of infection at the site of the blood draw, especially if you have a weakened immune system. Rarely, some people feel a little faint or lightheaded after a blood draw.
