A complete blood count (CBC) is a Blood Test that examines the numbers and features of blood cells. It examines three types of cells: red blood cells (RBCs), white blood cells (WBCs) and Platelets.
A CBC measures the following:
- White blood cell (WBC, leukocyte): count White blood cells to protect the body from infection. White blood cells attack and kill the bacteria, virus, or another organism that causes it when an infection develops. White blood cells are larger than red blood cells but less numerous. When a person has a bacterial infection, white cells rapidly grow. Often the number of white blood cells is used to detect infection or to see if the body is handling cancer.
- White blood cell types (WBC differential): Neutrophils, lymphocytes, monocytes, eosinophils, and basophils are the main forms of white blood cells. Another part of this study is immature neutrophils or band neutrophils. Each cell type plays a different role in safeguarding the body. The numbers of each of these white blood cell types provide essential immune system information. Too many or too few of the various forms of white blood cells may help identify an infection, an allergic or toxic reaction to drugs or chemicals, and other disorders, such as leukaemia.
- Red blood cell (RBC) count: Red blood cells carry oxygen from their lungs to the rest of their bodies. They even bring carbon dioxide back to their lungs so they can exhale it. If the amount of RBCs is small (anaemia), the body may not get the oxygen it needs. If the count is too high (polycythemia), the red blood cells can clump together and block tiny blood vessels. This also makes the transfer of oxygen to the red blood cells challenging.
- Hematocrit (HCT, packed cell volume, PCV): This check tests the number of red blood cells taking up space (volume) in the blood. The value is given in blood volume as a percentage of red blood cells. For example, a hematocrit of 38 means that red blood cells comprise 38 percent of the blood volume. Hematocrit and haemoglobin values are the two major tests that show whether there is anaemia or polycythemia.
- Hemoglobin (Hgb): The red blood cells are filled with the haemoglobin molecule. It carries oxygen and gives its red color to the blood cell. The haemoglobin test measures the amount of haemoglobin in the blood and is a good measure of the capacity of the blood to carry oxygen across the body.
- Red blood cell indices: There are three measures of the red blood cells: mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean concentration of corpuscular haemoglobin (MCHC). They are weighed by a computer, and their values derive from other CBC measurements. The MCV displays the red blood cells in proportion. The MCH value is the haemoglobin content of an average red blood cell. The MCHC tests haemoglobin levels in a typical red blood cell. Such numbers help to detect multiple forms of anemia. Red cell distribution width (RDW) may also be calculated, showing if the cells are all the same or of different sizes or shapes.
- Platelet (thrombocyte) count: The minor type of blood cell is the Platelets (thrombocytes). They are critical in clotting the blood. The Platelets swell when bleeding happens and clump together, forming a sticky barrier that helps stop bleeding. Uncontrolled bleeding can be a concern when there are too few Platelets. There is a risk of a blood clot forming in a blood vessel if there are too many Platelets. Platelets may also be involved in artery hardening (atherosclerosis).
- Mean Platelet volume (MPV): Mean number of Platelets measures the total sum (length) of Platelets. Mean Platelet volume is used to identify such diseases, along with Platelet count. If the Platelet count is usually the mean volume of the Platelet might still be too high or too low
Why is a CBC done?
A CBC is a standard Blood Test. It is often done as part of a routine checkup but can be done anytime. A CBC may be done to:
- Learn information about your general health
- Check how well the bone marrow and spleen are working
- Help diagnose diseases and conditions that affect blood cells, such as anaemia, infection, blood disorders or leukaemia
- Provide a baseline to compare with future CBCs
- Check for bone marrow suppression
- Monitor a condition (as a part of follow-up)
How is a CBC done?
A CBC is normally performed in a laboratory or hospital.
You may be given specific instructions to obey before getting a CBC completed. Some medicines influence the outcomes of CBC, so you might be told to stop taking some medicines before a CBC. Check with the laboratory to see if any drugs should be stopped and for how long.
Typically the blood is drawn from a vein in the arm. To apply pressure to the wound, an elastic band (tourniquet) is wrapped around your upper arm to make it easier to see the veins. You may be told to make a palm so that the veins would stand out. It cleanses and disinfects the skin. A needle is placed into the vein, thus removing a small amount of blood. You may experience a sensation of pricks or stingings.
The blood is collected in a tube and marked with your name and other details that marks you. Occasionally more than one blood tube is collected. Remove the tourniquet, and remove the needle. When the needle is taken off, you can experience slight Pain. The pressure is applied before bleeding ceases in the region where the needle was inserted. You put a small bandage on the area.
A medical expert (technologist) analyses the extracted blood using microscopes and other special equipment.
What do the results mean?
The results of CBC are given as numbers and sometimes rely on other factors like sex, age and history of the medication. To have significance, they should be compared with a standard reference range or with previous tests. Normal ranges for CBCs can differ slightly from laboratory to laboratory.
Usually, a CBC contains general information which can provide hints about potential health issues for doctors. Input from a CBC lets doctors determine if a diagnosis needs more testing or procedures. It can also help the doctor to establish or update treatment plans.
The best person to describe your CBC findings and what they mean for you is a doctor familiar with your medical background and general health.
Popular terminology for explaining CBC outcomes are:
- Anaemia – not enough healthy RBCs or haemoglobin
- Leukopenia -low number of WBCs
- Neutropenia – low number of neutrophils
- Leukocytosis – increased number of WBCs
- Thrombocytopenia – low number of platelets
- Thrombocytosis – increased number of platelets
The Complete Blood Count results can be high or low for several factors. Below are some examples of irregular cancer-related CBC findings.
Abnormal RBC counts
A low RBC count may be due to the following:
- Anaemia due to prolonged bleeding or blood loss (haemorrhage), a diet lacking iron or specific vitamins, certain types of chemotherapy, blood disorders or chronic disease
- Hodgkin Lymphoma and other lymphomas
- Cancers of the blood, such as leukaemia and multiple myeloma
- Some myeloproliferative disorders
A high RBC count may be due to the following:
- Dehydration, such as from severe diarrhoea
- Kidney tumors
- Lung diseases
- Polycythemia vera (a myeloproliferative disorder)
Abnormal WBC counts
A low WBC count may be due to the following:
- Viral infection
- Severe bacterial infection
- Bone marrow suppression caused by treatments like Chemotherapy or radiation therapy
- Bone marrow diseases, such as leukaemia or myelodysplastic syndrome (MDS) anemia
A high WBC count may be due to the following:
- Some myeloproliferative disorders
- Some types of cancer, such as bronchogenic carcinoma
- Certain drugs, such as colony-stimulating factors
- Stress, allergies or tissue injury
Abnormal Platelet counts
A low Platelet count may be due to the following:
- Some types of cancer, such as leukaemia or lymphoma
- Autoimmune diseases
- Bacterial infection
- Viral infection
- Chemotherapy or radiation therapy
- Having many blood transfusions
- Certain drugs, such as nonsteroidal anti-inflammatory drugs (NSAIDs), including acetylsalicylic acid (ASA, Aspirin, salicylate) and ibuprofen (Motrin, Advil, Nuprin)
A high Platelet count may be due to the following:
- Prolonged bleeding or blood loss (haemorrhage)
- Anaemia from low iron levels
- Infection (inflammation)
- Surgical removal of the spleen (splenectomy)
- Polycythemia vera
- Some types of leukaemia