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.
FAQs
A CBC is a commonly performed lab test. It can be used to detect or monitor many different health conditions. Your health care provider may order this test: If you are having symptoms, such as fatigue, weight loss, fever or other signs of an infection, weakness, bruising, bleeding, or any signs of cancer.
Does a complete blood count show everything? ›
A complete blood count, also called a CBC , usually doesn't give all the answers about a diagnosis. Results outside the expected range may or may not need follow-up. A health care provider might need to look at the results of other tests as well as the results of a CBC .
How accurate is complete blood count? ›
Full blood count and accuracy
The full blood count test is not foolproof and errors sometimes occur. If this happens, your doctor will want to repeat the test. Errors may include: failure of the equipment – for example, the blood clots in the vial.
How to pass a CBC test? ›
Make sure to wear a short-sleeved shirt or a shirt with sleeves that you can easily rollup. You can typically eat and drink normally before a CBC. However, your doctor may require that you fast for a specific amount of time before the test. That's common if the blood sample will be used for additional testing.
Is a CBC a good indicator of health? ›
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.
What if CBC test is not normal? ›
A CBC counts the cells in your blood. There are many reasons your levels may not be in the normal range. For example: Abnormal levels of red blood cells, hemoglobin, or hematocrit may be a sign of anemia, heart disease, or too little iron in your body.
What diseases can a CBC detect? ›
Some of the most common diseases a CBC detects include anemia, autoimmune disorders, bone marrow disorders, dehydration, infections, inflammation, leukemia, lymphoma, myeloproliferative neoplasms, myelodysplastic syndrome, sickle cell disease, thalassemia, nutritional deficiencies (e.g., Iron, B12 or folate), and ...
Can blood test detect all problems? ›
Blood tests can be used to help a doctor identify a variety of health conditions, including infections, anemia, high cholesterol, vitamin deficiencies, organ failure, HIV, cancer, diabetes, and more.
What cancers can a complete blood count detect? ›
Complete blood count (CBC).
A CBC can detect some blood or immune system cancers like leukemia and lymphoma. But it can't detect solid organ cancers like lung, breast or colon cancers. It can tell your provider how your body responds to treatment and whether the cancer has spread to your bone marrow.
Can CBC results be wrong? ›
The accuracy of lab results can be affected by many factors, like testing conditions, testing facilities, and the test itself. While it's impossible for lab results to be accurate 100% of the time, you can take steps to increase the chances that the results are right.
Limitations. Some medical conditions or problems with the blood sample may produce inaccurate results. If the sample is visibly clotted, which can be caused by poor phlebotomy technique, it is unsuitable for testing, because the platelet count will be falsely decreased and other results may be abnormal.
What is the difference between a full blood count and CBC? ›
A full blood count (FBC) is a very common blood test. It's also sometimes called a complete blood count (CBC) or full blood examination (FBE). A full blood count can provide information about a range of conditions.
What not to do before CBC test? ›
Avoid drinking or eating anything for 8-12 hours before the test. You may drink only water. You should not eat 3 hours before the clinical blood test. Eat less fatty and fried food, and avoid alcohol 1-2 days prior to the test.
Does drinking water affect CBC? ›
The patient's hydration status significantly alters CBC values.
How can I improve my CBC results? ›
Nutrient-Rich Diet: Opt for a balanced diet abundant in iron, vitamin B12, folate, and antioxidants. Foods like spinach, lentils, lean meats, fish, nuts, and citrus fruits are excellent choices. Iron is essential for RBC production, while B12 and folate aid in their maturation.
What Cannot be detected by a CBC? ›
Healthcare providers categorize cancer blood tests into four main types: Complete blood count (CBC). A CBC can detect some blood or immune system cancers like leukemia and lymphoma. But it can't detect solid organ cancers like lung, breast or colon cancers.
What is the disadvantage of CBC test? ›
Limitations and Potential Drawbacks
CBC tests provide general information about blood components but do not provide a specific diagnosis for a particular condition. Further tests and investigations are often required to confirm or narrow down potential health concerns.
Can you be sick and have normal blood work? ›
If you are feeling unwell, then it is best to wait until you feel better before you take your test. This is because illnesses such as colds and flu can affect your blood test results and cause short-term changes which might lead you to be anxious for no reason. They also don't reflect what is normal for you.
How often should I get a CBC done? ›
Many diseases and conditions can affect your blood. But these disorders may show only minimal symptoms or, in some cases, no symptoms at all. The first sign of an abnormality may appear on the complete blood count. For that reason, it's important that everyone have this test once a year.
