What Is a Complete Blood Count (CBC) Test?
The complete blood count (CBC) is one of the most commonly ordered blood tests. To understand this test, it is important to know that blood consists of two major parts: plasma and cellular elements. The plasma is the part of the blood that is liquid which allows the blood to flow easily. The other part of the blood consists of blood cells.
The major cells in the blood are white blood cells (WBC), red blood cells (RBC), and platelets. Each of these types of cells carries out specific and important functions.
The complete blood count test measures the quantity of all the different types of cells in the blood. It also provides some valuable information on other parameters related to each type of blood cell.
Complete Blood Count Procedure
The complete blood count may be done in many different health care settings including doctors' offices, clinics, urgent care facilities, emergency rooms, hospitals, and outpatient medical laboratories.
The complete blood count test is performed by drawing a few milliliters (one to two teaspoons) of blood from a vein. Most commonly, the sample is obtained from a vein that is visible from the skin, such as a vein on the back of the hand or the inner angle of the elbow (antecubital fossa).
A tourniquet is usually applied to the area proximal to the vein (closer to the center of the body than the vein itself). This technique will make the vein more visible and plump by limiting the blood from the vein going back toward the heart. The tourniquet is only applied for a brief period of time (a few minutes at the most) and it is removed as soon as blood is drawn.
The skin overlying the vein is cleaned using an alcohol pad, and then a needle is inserted through the area of cleansed skin into the vein below where the tourniquet is applied. The blood is then pulled from the vein via the needle by gently pulling the plunger on the syringe or by a connection of the needle to a special vacuum vial that collects the blood.
This sample is then taken to the laboratory for analysis, and the complete blood count results may be available within hours after collection. Prompt delivery of the blood sample to the laboratory for analysis is important. A sample that is not delivered in a timely manner may yield inaccurate results.
Components of Complete Blood Count
The complete blood count measures many important values pertaining to the blood cells. The complete blood count interpretation is done by doctors who review the results of the testing.
The complete blood count generally includes the following components:
- White blood cell count (WBC or Leukocyte count)
- WBC differential count
- Red blood cell count (RBC or erythrocyte count)
- Hematocrit (Hct)
- Hemoglobin (Hbg)
- Mean corpuscular volume (MCV)
- Mean corpuscular hemoglobin (MCH)
- Mean corpuscular hemoglobin concentration (MCHC)
- Red cell distribution width (RDW)
- Platelet count
- Mean platelet volume (MPV)
The main components are the blood cells such as white blood cells, red blood cells, and platelets. The other components represent additional information about these cells including their size, color, function, and maturity.
The white blood cell (WBC) differential refers to number of the different types of white blood cells seen in the blood. The different types of WBCs that have specific functions that are routinely reported in a complete blood count are neutrophils, lymphocytes, basophils, eosinophils, and monocytes.
Analysis of the Complete Blood Count
The blood sample drawn for a complete blood count is analyzed in a medical laboratory. The complete blood count analysis is routinely and reliably done by automated machines in most laboratories. A small sample of the blood drawn from a person is fed into the machine and within a few minutes, the values of the components of the complete blood count are displayed and printed for review. This is called an automated cell count and differential.
The conventional method to analyze these data is to obtain a small sample of the collected blood and place it on a glass slide for visual review under a microscope. This is usually done by a trained laboratory technologist or a doctor. This method is still widely used when results of a complete blood count need further review to confirm certain abnormal values, or a doctor wants to see how the blood cells look (for example, if any abnormal features are present which would not be reported by an automated complete blood count). This is called the manual differential analysis.
Values for Components of Complete Blood Count
The complete blood count values are usually reported based on the number of cells in a specific volume of blood. The normal values may differ slightly based on the reference range and the machine used in the laboratory and, therefore, the results may be slightly different from one laboratory to the next. The normal reference range is typically provided and printed with the results of the complete blood count for accurate interpretation. Different laboratories may report slightly different reference ranges.
The following lists some of the typical values of the components of the complete blood count:
- WBC (white blood cell) count signifies the number of white blood cells in the blood and usually ranges between 4,300 and 10,800 cells per cubic millimeter (cmm).
- RBC (red blood cell) count measures the number of red blood cells in a volume of blood and usually ranges between 4.2 to 5.9 million cells per cmm.
- Hemoglobin (Hbg) measures the amount of the hemoglobin molecule in a volume of blood and normally is 13.8 to 17.2 grams per deciliter (g/dL) for men and 12.1 to 15.1 g/dL for women.
- Hematocrit (Hct) signifies the percentage of the whole blood occupied by red blood cells and usually ranges between 45%-52% for men and 37%-48% for women.
- Mean corpuscular volume (MCV) is the measurement of the average size or volume of a typical red blood cell in a blood sample and usually ranges between 80 to 100 femtoliters (a fraction of one-millionth of a liter).
- Mean corpuscular hemoglobin (MCH) measures the amount of hemoglobin in an average red blood cell and usually ranges between 27 to 32 picograms (a small fraction of a gram).
- Mean corpuscular hemoglobin concentration (MCHC) measures the average hemoglobin concentration in a volume of blood, and it usually ranges between 32%-36%.
- Red cell distribution width (RDW) measures the variability in the red blood cells' size and shape and usually ranges between 11 to 15.
- Platelet count measures the number of platelets in a volume of blood and usually ranges between 150,000 to 400,000 per cmm.
- Mean platelet volume (MPV) measures the average size of platelets in a volume of blood. The normal range is between 6 to 12 femtoliters (a very small fraction of a liter).
Function of Cells in a Complete Blood Count
The cells in a complete blood count serve very important functions in the body.
The white blood cells are an important component of the immune system which fights against infections in the body. They are made in the bone marrow and undergo a complex series of steps to gain functional maturity at which time they are released into the blood stream to perform their function. An elevated WBC count typically indicates some kind of infection or inflammation in the body. Each of the cells in the WBC differential also has specific functions that are important to note when analyzing the results of a complete blood count. For example, eosinophils may be involved in allergic reactions. Neutrophils are usually more suggestive of a bacterial infection whereas lymphocytes typically suggest a viral infection. Physicians will sometimes refer to a left shift on the differential which means that the neutrophils are elevated or they may refer to a right shift which indicates that the lympocytes are elevated.
Red blood cells are a vital part of the oxygen transportation system in the body. The hemoglobin molecule is a complex protein structure that exists within the red blood cells and is the physical carrier of oxygen from the lungs to all other parts of the body. Decreases in the red blood cell count or the hemoglobin level may interfere with the oxygen carrying capacity of the red blood cells. A diagnosis of anemia indicates a low number of RBCs or a low level of Hgb.
Platelets are an important part of the blood clotting system. They are not complete cells, but fragments of larger cells called megakaryocytes. Platelets become activated when there is any evidence of bleeding or injury somewhere in the body. They clump together at the site of bleeding (called platelet aggregation) in an attempt to plug up the bleeding site. This is done in concert with other components of the clotting system which includes some specific proteins such as thrombin.
Why Are Complete Blood Count Tests Used?
Uses of the complete blood count are extensive. In general, the complete blood count can be done as part of routine health exam and general screening by a doctor. It may be ordered if an infection or anemia is suspected. It may also be ordered to evaluate abnormal bleeding.
As mentioned earlier, an elevation of the white blood cell count or an abnormality of the white blood cell differential may be suggestive of an infection or inflammation. A high or a low white blood cell count could also be a sign of underlying cancer such as leukemia or lymphoma.
A low red blood cell or hemoglobin count typically indicates anemia (low blood). Anemia, typically seen as low hemoglobin or low hematocrit on the complete blood count, is a sign of an underlying disease and it is not a disease itself. Anemia can have many causes including blood loss, bone marrow problems, nutritional deficiencies, genetic hemoglobin structural or functional problems (sickle cell or thalassemia), or kidney failure. These are only the most common causes of anemia, and the list of all causes of anemia is very extensive. Anemia found in a complete blood count may be suggestive of ongoing slow blood loss and, therefore, can be used to detect cancers, such as colon cancer. If anemia is detected, usually the MCV and RDW give some additional clues as to the possible causes of anemia.
A low platelet count (thrombocytopenia) may also be detected in the complete blood count. This may be due to bone marrow problems, some medications or excessive alcohol use, immunologic or genetic problems, advanced liver disease, or cancers such as leukemia. The MPV may indicate how rapidly platelets are made in the bone marrow and released into the blood stream. A high platelet count may also be suggestive of an inflammation or blood malignancy, such as leukemia and lymphoma.
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care
Bonilla, Francisco A, MD, PhD, and E Richard Stiehm, MD. "Laboratory evaluation of the immune system." UpToDate.com. Updated Dec 2, 2015.