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Understanding What is CBC Blood Test

A complete blood count (CBC) is a blood test that measures the amounts and sizes of different types of blood cells in a sample of your blood, including red blood cells, white blood cells, and platelets.

Purpose of the CBC Blood Test

A CBC blood test can give your doctor a lot of information about your health. Your healthcare provider can use this information to diagnose medical conditions, monitor known health conditions, and monitor your response to treatments. 

Abnormal results on a CBC test can lead to a diagnosis of a wide range of health conditions, including nutritional deficiencies, infections, heart or lung disease, liver damage, autoimmune diseases, and blood and bone marrow disorders.

Common Uses in Medical Diagnosis

A CBC blood test is a commonly performed lab test for routine health screening. It is also done to identify the cause of certain symptoms, to monitor known medical conditions, and to monitor response to treatment and medications.

Components of a CBC Blood Test

A complete blood count measures the following:

Red Blood Cells (RBCs)

• Red blood count (RBC) count: Number of red blood cells in a fixed amount of blood, usually 1 microliter (mcL) of blood. 

• Hemoglobin: Amount of hemoglobin in blood. Hemoglobin is an iron-containing protein in red blood cells that carries oxygen from the lungs to the rest of the body.

• Hematocrit: Percentage of blood consisting of RBCs.

• Mean corpuscular volume (MCV): Average size or red blood cell volume.

• Mean corpuscular hemoglobin (MCH): Average concentration of hemoglobin per red blood cell.

• Mean corpuscular hemoglobin concentration (MCHC): Average concentration of hemoglobin in a fixed amount of blood.

• Red cell distribution width (RDW): Difference in size between the smallest and largest red blood cells. A low number means all RBCs are of similar size. A high RDW means there is a big difference in size between the smallest and biggest RBCs. 

White Blood Cells (WBCs)

A CBC test measures the total white blood cell counts. CBC with differential measures the amount of each different type of white blood cell.

• White blood cell (WBC) count: Total number of white blood cells in a fixed amount of blood.

• Neutrophils: These are a type of white blood cell that are first responders for fighting bacterial and fungal infections. 

• Monocytes: These are a type of white blood cell that alert other WBCs about bacteria and viruses and mop up debris from infections.

• Lymphocytes: T-cell and B-cell lymphocytes fight viral infections and help the immune system remember previous harmful organisms and more efficiently fight infections in the future.

• Eosinophils: These WBCs fight parasitic infections and their number increases in allergic reactions.

• Basophils: These WBCs release histamine in allergic reactions and heparin to prevent blood clots from forming. 

• Immature granulocytes: White blood cells that are not fully developed.

• Nucleated red blood cells (nRBC): Immature red blood cells. 

Platelet Count

• Platelet count: Number of platelets in a fixed amount of blood. 

• Mean platelet volume (MPV): Average platelet size.

Why Do I Need a Complete Blood Count?

Routine Check-Ups vs. Diagnostic Purposes

CBC can help to diagnose a wide range of conditions. It is commonly done as part of a routine health checkup. It is also done to monitor health during pregnancy. In addition, healthcare providers use CBC for diagnostic purposes to find the cause of certain symptoms.

Common Medical Symptoms Leading to CBC Testing

Your doctor may order a complete blood count if you are complaining of:

• Fatigue

• Weakness

• Unexplained weight loss

• Fever or other signs of infection

• Excessive bleeding, fainting, unusual bruising

• Signs of cancer such as night sweats, weight loss, fatigue, fever

Assessing and Monitoring Health

Doctors also use CBC to monitor the health of people with conditions such as chronic kidney disease or cancer. This blood test is also done to see how well a treatment is working or how a medication may be affecting your body. For example, certain treatments can cause leukopenia (low white blood cell count) and weaken your immune system.

How is the CBC Blood Test Done?

Test Preparation 

There is no special preparation for a CBC. You do not need to fast overnight. However, if you are having other blood tests done along with CBC, you may need to prepare for them, for example, by fasting for 8-12 hours, as instructed by your healthcare provider.

How the Test is Performed

A complete blood count (CBC test) is performed on a blood sample. A laboratory technician will draw blood after they clean the area and insert a needle into a vein in your arm or hand. They will collect a blood sample in one or more test tubes. Babies usually have the blood drawn from a vein in the heel. The blood sample is then sent to the laboratory for analysis.

Check out tips to avoid side effects when having blood drawn. 

Possible Risks and Discomfort

You may feel a slight pinch when the needle is inserted into your vein. It’s only a prick and the discomfort is minimal. The lab tech will place a bandage on the area. It may feel slightly sore for a few hours with some minor bruising. Keeping the self-adherent bandage (wrap) on for a few hours and avoiding heavy lifting can help to prevent or minimize bruising. 

CBC is a common and safe blood test. However, there is a small risk involved of infection from the needle stick.

Interpretation of CBC Test Results

What Is The Normal CBC Range?

Note: The reference range for CBC varies based on the laboratory performing the test. Also, the normal range varies with age, gender and hormone status, as well as pregnancy and other health conditions. 

Make sure you compare the normal value ranges in similar units. For example, the normal range can look very different if a laboratory reports the results in k/mcL or the number of cells multiplied by 1,000 per microliter.

Learn more about how to read blood test results.

What Would Show Up In a CBC Blood Test?

A wide range of medical conditions can show up in a CBC blood test, ranging from nutritional deficiencies like iron deficiency anemia to more serious health problems like bleeding diseases and cancer.

What Problems Can a CBC Detect?

CBC can detect and monitor a broad range of health conditions. An abnormal blood count (high or low platelet count, red blood cell count, or white blood cell count) may be due to blood diseases, immune system conditions, infections, nutritional deficiencies, cancer, and other medical conditions.

What Do Abnormal CBC Results Indicate?

Possible Causes of High Red Blood Cell Count (Erythrocytosis)

• Rare blood disorders like polycythemia vera, in which an increased number of red blood cells make the blood thicker and increase the risk of blood clots.

• Heart disease, such as congenital heart disease and congestive heart failure.

• Lung diseases such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis.

• Hemoglobinopathies, which are disorders in which there is an abnormal amount or structure of hemoglobin.

• Sleep apnea.

• Smoking.

• Living at high altitudes.

Potential Causes of Low RBC Count (Anemia)

• Nutritional deficiencies like vitamin B12 deficiency or iron deficiency.

• Bleeding or blood loss from injury, surgery, or other causes like bleeding tumors.

• Blood and bone marrow disorders such as myelodysplastic syndrome or bone marrow failure.

• Certain cancers, such as leukemia, lymphoma, and multiple myeloma.

• Radiation and/or chemotherapy to treat cancer disease.

• Certain medicines such as aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, and antibiotics.

Possible Causes of High WBC Count (Leukocytosis)

• Viral, fungal, and bacterial infections.

• Allergies (hay fever or seasonal allergies).

• Allergic conditions such as asthma.

• Severe allergic reactions, including drug reactions.

• Autoimmune disorders, such as rheumatoid arthritis, in which the body has an abnormal immune response.

• Bone marrow diseases such as myelofibrosis.

• Immune system problems.

• Burns.

• Cancers such as leukemia and lymphoma.

• Pregnancy.

• Smoking.

Possible Causes of Decreased WBC Count (Leukopenia)

• Bone marrow disorders such as aplastic anemia.

• Cancers such as multiple myeloma.

• Autoimmune diseases such as rheumatoid arthritis and lupus.

• Infections such as viral hepatitis and human immunodeficiency virus (HIV).

• Cancer treatment such as chemotherapy and radiation therapy.

• Certain medications, such as antipsychotic drugs and medications used to treat hyperthyroidism (overactive thyroid).

Potential Causes of High Platelet Count (Thrombocytosis)

• Reactive thrombocytosis, caused by blood loss, infections, removal of the spleen, iron deficiency, hemolytic anemia, inflammatory disorders, surgery, and trauma.

• Essential thrombocythemia due to alterations in certain genes where the bone marrow forms too many cells that mature into platelets.

Possible Causes of Low Platelet Count (Thrombocytopenia)

• Enlarged spleen.

• Thrombocytic thrombocytopenic purpura, which is a rare condition in which platelets get used up in large numbers because small blood clots suddenly form throughout the body.

• Hemolytic uremic syndrome, a condition in which RBCs are destroyed and platelet levels fall, affecting kidney function.

• Certain types of anemia.

• Viral infections like hepatitis and HIV.

• Leukemia and other cancers.

• Radiation therapy and chemotherapy.

• Autoimmune disorders like lupus and rheumatoid arthritis.

• Heavy alcohol use.

• Pregnancy.

Follow-Up Steps After Receiving Results

Your healthcare provider will discuss further steps after the results of your CBC are back. You may need monitoring, further testing, or treatment for a medical diagnosis.

Limitations and Related Tests

Does CBC Test Include Liver and Kidney Functions?

CBC test does not include liver and kidney functions. However, the results of this test can indirectly tell your healthcare provider about how well your kidneys and liver are functioning or point towards certain liver and kidney diseases. Your healthcare provider can order liver function tests and kidney function tests to evaluate these organs more closely.

Comparison of CBC with Other Diagnostic Tests

CBC is a commonly performed blood test that provides an overview of blood health and overall health. Abnormal results can indicate physical stress, tissue damage, or a blood disease. Other tests can provide more specific information about organ systems and medical conditions. For example, if blood count results show increased white blood cells, your doctor may suspect bone marrow or cancer disease and order further testing such as imaging studies, bone marrow aspiration, or biopsy.

Related Blood Tests and Their Importance

As noted, complete blood count (CBC) is a common blood test. Other blood tests commonly done with CBC include:

• Basic metabolic panel that measures glucose, calcium, electrolytes, BUN, and creatinine levels in the blood and provides information about the heart, liver, and kidneys.

• Lipid panel to measure blood cholesterol levels.

• Blood enzyme levels to detect heart attacks (troponin and creatine kinase) and muscle damage (CK-MB).

Coagulation panel (blood clotting tests) to detect bleeding disorders and monitor people who take anticoagulants (blood thinners).