What Is a CBC (Complete Blood Count)?
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Medically reviewed by Rafle Fernandez, MD
A “complete blood count,” (CBC) is a group of commonly performed tests which use a sample of your blood. It counts the number of different types of cells and components in your blood, like red blood cells. It also shows how much hemoglobin is in your blood, the protein that red blood cells use to carry oxygen around the body.
Your primary care provider, emergency room caregiver, hospitalist physician, or specialist might ask you to get a CBC test. It can provide information that helps them identify and diagnose multiple different diseases, if needed, including anemia, blood cancers, infections, autoimmune disorders, and blood loss.
Purpose
Because blood is so critical for sustaining life, it’s not surprising that a CBC is an important test for diagnosing many different medical conditions. A CBC analyzes multiple components of your blood, including:
Red blood cells, which carry needed oxygen to all the cells of your body.
White blood cells, a group of various kinds of cells of the immune system that are important for fighting infection
Platelets, cell fragments which are important for healthy blood clotting
A CBC is a relatively inexpensive test which gives a lot of basic information. So, in many situations, it’s one of the first tests your healthcare provider might perform if your diagnosis isn’t clear from your medical history and exam alone.
For example, a CBC might be helpful in scenarios such as:
You are pale and experiencing a lot of fatigue, and your primary care provider suspects anemia, a condition characterized by not having enough healthy red blood cells.
You have fever and other symptoms that can indicate some kind of infection, like pneumonia (lung infection).
You are having a lot of problems with excess bleeding, like lots of nosebleeds.
You seem to be getting a lot of infections, indicating a possible problem with your immune system.
You have some signs of chronic or sudden bleeding, like dark, tarry stools or bright red blood in your stool.
You have symptoms of possible blood cancer, like fatigue, increased bruising, and frequent infections.
Sometimes, a CBC along with a physical exam and medical history provides enough information for a diagnosis. However, a CBC is often paired with other tests that can give complementary information.
CBCs can helpful for monitoring as well. For example, if you’ve been given blood after a traumatic injury, a CBC can show if you now have enough red blood cells and hemoglobin in your body. Or your healthcare provider might get a CBC to see how your anemia treatment is working.
However, unless you have symptoms or signs of specific medical conditions, your healthcare provider will probably not recommend a CBC for screening or general tests of your health.
Components of a CBC
A CBC analyzes several different properties of your blood, giving information about red blood cells, platelets, and white blood cells.
Red Blood Cells and Platelets
A CBC provides a lot of specific information about your red blood cells and platelets, including.
Red blood cell (RBC) count: An RBC count tells you the number of red blood cells within a certain volume (amount) of blood.
Hemoglobin (Hb): The hemoglobin value tells you the weight of total hemoglobin found in a certain amount of blood.
Hematocrit (HCT): Hematocrit gives the percentage of red blood cells in a certain volume of blood.
Reticulocyte count: This is sometimes included as part of a CBC. It gives the number of slightly immature red blood cells in your sample.
Platelets: The platelet value tells you how many platelets you have in a certain volume of blood.
A subset of the CBC tests give somewhat overlapping information about the size and shape of RBCs and amount of hemoglobin they contain. These are called red blood cell indices, described below:
Mean corpuscular volume (MCV): This tells you the average size of your RBCs.
Mean corpuscular hemoglobin (MCH): This shows the average amount of hemoglobin in a single RBC.
Mean corpuscular hemoglobin concentration (MCHC): This tells you how much hemoglobin is in a specific volume of red blood cells after they have been packed together.
Red cell distribution width (RDW): This tells you if your RBCs differ in volume and size throughout your blood sample.
White Blood Cells
A CBC also tells you about your white blood cells. White blood cell (WBC) count is an important part of the CBC. It tells you the number of white blood cells within a certain volume of your blood.
Although not technically part of a CBC, something called a “differential” is also often done along with it. The differential gives information about the relative percentages of different kinds of immune cells that were found in your WBC. Sometimes this gives clues about the type of infectious organism that might be present. You may see this listed on your medical chart as a "CBC with differential."
How Does a Complete Blood Count Work?
To get a CBC, someone will have to draw a sample of your blood. It’s not a test that requires a lot of preparation or recovery.
Before the Test
Getting a CBC only requires a few minutes. A nurse or a phlebotomist (health professional specializing in drawing blood) will typically do it in your healthcare provider's office or a lab service center.
Typically, you sit during the blood draw. The provider looks for a vein on your arm, or another location if you don’t have a usable arm vein. They may wrap a tight band around your arm while you squeeze your fist, to make the vein pop out more. Next, they clean the area over the vein.
During the Test
When you are ready, the provider will insert a needle into your vein while you stay very still. This hurts a bit, but usually only for a moment. Breathe deeply and remain calm. You can look away if you don’t like the sight of blood.
Blood flows out of the vein into a small collection tube. It usually only takes a few seconds to fill the tube, but you might need more than one tube filled. But it’s much less blood than you’d give if you were donating blood.
After enough blood is collected, the provider removes the needle, which also might hurt a bit. Then they bandage the small entry point of the needle.
After the Test
If you are feeling woozy from the blood draw, stay seated a while. Someone can bring you water and perhaps something to eat. When you are feeling better, you can go about your everyday activities as normal.
You can’t get your CBC results immediately. Someone sends the blood sample to a lab. Typically, a laboratory technician there uses an automated machine to calculate your results.
Risks and Precautions
Getting a CBC carries little risk. You might get a little bruising where you had the blood draw, and the area may be a little sore. But symptoms go away quickly for most people. Theoretically, a blood draw also has a slight risk of causing an infection, because your skin is broken. However, this risk is extremely small.
How to Prepare for a CBC
You might get blood for a CBC taken at your primary care provider’s office, at a lab that your provider recommends, or at the hospital.
There isn’t much you need to do to prepare. Wear something that will give easy access to your arm, like a shirt with loose sleeves. Bring your ID, an insurance card if you have one, and any information given to you by your medical provider.
Most of the time, you won’t need to fast before you get your blood drawn. However, often your healthcare provider will have you get a CBC at the same time as other tests. For some of these, you might need to fast. Similarly, you usually won’t have to stop taking any medications, but ask before you come. Some people feel a little dizzy or light-headed after a blood draw, so it doesn't hurt to bring a light snack with you, such as some fruit or a granola bar.
Compared to some other tests, a CBC is relatively inexpensive. If you are self-paying and getting it as an outpatient, it should be less than $100 and perhaps considerably less. However, your charges will be greater if you are getting it at the same time as other tests.
Results
Depending on the context, results from your CBC might be available within a few hours or in a day or so. Your healthcare provider might review them with you at a follow up visit or phone call, or you might look them up yourself online.
Interpreting Your Results
The CBC doesn’t give a simple “positive” or “negative” result. Instead, you’ll get specific numbers for each specific CBC value. Your healthcare provider helps interpret these results in the context of your whole clinical picture.
Your results should provide a range of normal values as context for viewing your specific test results. Abnormal results (those out of the expected range) are typically the ones most important to consider.
However, in some circumstances having a value outside of this range won’t mean anything important, especially if the value is only slightly out of range. Sometimes normal values vary by your age, gender, or other factors. For example, a normal hemoglobin value for women is about 12 to 15 gm/dL, but for men it is around 14 to 17.
The following are some examples of some abnormal CBC test results and what they might mean.
Abnormal RBC or Platelet Findings
If your RBC count, hematocrit, or hemoglobin tests are lower than normal, it might indicate chronic anemia. However, it might be a sign of other health problems, like an active bleed, blood cancer, or kidney failure.
If your RBCs are higher than normal, it might indicate dehydration or a condition called polycythemia vera, in which your body makes too many red cells.
Your high or low red blood cell results can also help with diagnosing an underlying condition. For example, different kinds of anemia cause your RBCs to have a different size, and they might show differences in the reticulocyte count. Thus, the CBC can give clues about anemia from iron deficiency versus anemia from another cause.
If your platelets are lower than normal, it might indicate an inherited genetic problem or perhaps an autoimmune disorder. It may also be a response to certain drugs or medicine.
Higher than normal platelets might happen due to certain infections, certain medicines, or some bone marrow diseases.
Abnormal WBC Findings
If you have a lower than normal WBC count, it might be a sign of decreased production of white blood cells in your bone marrow, such as from a blood cancer or from a genetic immune condition. Certain drug side effects can also cause this, such as chemotherapy. It might also appear if you have an autoimmune disease.
A higher WBC count can be a sign of infection, especially one from bacteria. Sometimes it might indicate inflammation, which can occur due to a variety of medical condition, like rheumatoid arthritis. Stress, recent surgery, and smoking are more common causes. Less commonly, it might be a sign of cancer.
Sometimes the differential also gives clues as to what you're experiencing. For example, a parasitic infection might cause an increased percentage of a type of WBC called eosinophils.
Hard to Interpret Results
Sometimes, errors appear on CBC results (e.g. from some sort of laboratory error). If your results are way out of the expected range, your healthcare provider may ask you to get another CBC to confirm the validity of your results.
If your diagnosis still isn’t clear and you have unusual CBC results, a hematologist (a healthcare provider specializing in blood disorders) may need to provide additional expertise and perhaps perform additional tests.
A Quick Review
A CBC is a very common set of tests performed on a sample of your blood. It gives some basic information about the components of your blood, including red blood cells, white blood cells, and platelets.
A wide range of healthcare providers might use a CBC to help diagnose conditions like anemia, blood loss, infection, and blood cancers. The test is relatively inexpensive and doesn’t require a lot of preparation.
Your healthcare provider will use information from the CBC in combination with your other health information help determine a diagnosis, but you may need follow-up tests to truly identify what is going on.
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