Complete Blood Count (CBC)

What is a complete blood count (CBC)?

A complete blood count (CBC) is a blood test that providers use to monitor or diagnose health conditions. It can give your provider information about how medications or medical conditions are affecting your body, and about the health of your immune system. It can detect blood cancers, anemia, infections and other conditions.

For a CBC blood test, a healthcare provider takes a sample of your blood and sends it to a lab. The lab measures the amount of red blood cells, hemoglobin (the protein that carries oxygen in your red blood cells), white blood cells and platelets. They also measure the size of your blood cells. Along with a CBC, your provider might order a peripheral blood smear, which gives them more information about how your blood cells look under a microscope.

What’s a CBC with differential?

A CBC with differential means that the lab also counts each different type of white blood cell and immature (not-yet-fully-developed) blood cells. This gives your provider more information about what’s happening with different immune system cells.

When do you need a CBC blood test?

Your provider might order a CBC test:

• As part of a routine health exam.
• To investigate unexplained symptoms, like fever, unintended weight loss, night sweats, bruising, bleeding or fatigue.
• To monitor an ongoing health condition, like cancer or chronic kidney disease (CKD).
• To monitor how well treatment is working or how a medication is affecting your body. For instance, certain treatments can lower your white blood cell counts and weaken your immune system.
• To monitor your health while you’re pregnant.

What does a CBC blood test check for?

A CBC test can tell your provider if you have:

• Anemia (low levels of red blood cells or hemoglobin).
• Erythrocytosis (high concentrations of red blood cells).
• Leukocytosis (high white blood count).
• Leukopenia (low white blood count).
• Thrombocytosis (high platelet count).
• Thrombocytopenia (low platelet count).

These can be signs of many health conditions, including infections, cancer, blood conditions and medication side effects. But your provider can use the results of your CBC, along with your symptoms, to narrow down or diagnose the cause.

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.

What is normal CBC count range?

Normal ranges for values on your complete blood count can vary some, but in general, they include:

Normal ranges for a CBC blood test can vary based on:

• The lab performing the test.
• Your hormone status. Some references have male and female reference ranges, based on typical levels of hormones by sex. This will be different if you’re taking hormones.
• Your age. Adults and kids have different normal ranges.
• Whether you’re pregnant or have other health conditions.
• The units of measurement the lab uses. For instance, some ranges can look very different depending on whether the results are reported in cells per microliter (abbreviated mcL, uL or µL) or cells per microliter multiplied by 1,000 (abbreviated k/mcL, k/uL or k/µL). Measurements in cells per microliter will look much larger.

Always be sure to check the reference range (the range of numbers considered normal) provided on your results and compare your numbers only to the reference. Ask your provider if you have any questions.

When should I know the results of a CBC test?

Results are usually ready within a few days. Sometimes, it only takes 24 hours to get results. Your provider will contact you to explain the results and discuss next steps.

What do abnormal CBC results mean?

Many health conditions can cause high or low blood counts. Some of these are serious and others are common and easily treatable. For example:

• High red blood cell counts: Causes include heart and lung conditions, polycythemia vera and sleep apnea.
• Low red blood cell counts (anemia): Causes include nutritional deficiencies (like B12 deficiency or iron deficiency), blood loss/bleeding, low kidney function and blood and bone marrow disorders.
• High platelet counts: Causes include infections and immune system issues.
• Low platelet counts: Causes include immune thrombocytopenia, certain cancers and hemolytic uremic syndrome.
• High white blood counts: Causes include infections, allergic conditions, autoimmune conditions, lymphoma and leukemia.
• Low white blood counts: Causes include autoimmune diseases, medications, bone marrow disorders and infections.

If the results are abnormal what are the next steps?

If your results are out of the reference range, talk to your provider about what they might mean. They may order follow-up tests or start a treatment plan. Or you may not need treatment, only monitoring.

When should I call my doctor?

Contact your provider if you have any questions about the test or the results.

Does CBC show kidney function?

A CBC doesn’t directly show how well your kidneys are functioning. But some numbers, like lymphocytes and MCV, along with other tests, can help your provider get a better picture of how well your kidneys are working.