Complete Blood Count (CBC)

Definition of Complete blood count:
Complete blood count (CBC), also called full blood count (FBC) or blood panel, is a blood test used to evaluate your overall health and detect a wide range of disorders, including anemia, infection and leukemia. The complete blood count is the calculation of the cellular (formed elements) of blood. These calculations are generally determined by special machines that analyze the different components of blood in less than a minute. A complete blood count (CBC) gives important information about the kinds and numbers of red blood cells, white blood cells, and platelets in the blood.

What are values for a complete blood count (CBC)?
A complete blood count test measures several components and features of your blood, including:

White blood cell count (WBC): is the number of white blood cells in a volume of blood. Normal range varies slightly between laboratories but is generally between 4,300 and 10,800 cells per cubic millimeter (cmm). This can also be referred to as the leukocyte count and can be expressed in international units as 4.3 to 10.8 x 109 cells per liter.

White blood cell types (WBC differential): The major types of white blood cells are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Immature neutrophils, called band neutrophils, are also part of this test. Each type of cell plays a different role in protecting the body. The numbers of each one of these types of white blood cells give important information about the immune system. Too many or too few of the different types of white blood cells can help find an infection, an allergic or toxic reaction to medicines or chemicals, and many conditions, such as leukemia.

Red blood cell (RBC) count: Red blood cells carry oxygen from the lungs to the rest of the body. They also carry carbon dioxide back to the lungs so it can be exhaled. If the RBC count is low (anemia), the body may not be getting the oxygen it needs. If the count is too high (a condition called polycythemia), there is a chance that the red blood cells will clump together and block tiny blood vessels (capillaries). This also makes it hard for your red blood cells to carry oxygen.

Hemoglobin (Hgb): The hemoglobin molecule fills up the red blood cells. It carries oxygen and gives the blood cell its red color. The hemoglobin test measures the amount of hemoglobin in blood and is a good measure of the blood's ability to carry oxygen throughout the body.

Hematocrit (Hct): This is the ratio of the volume of red cells to the volume of whole blood. Normal range for hematocrit is different between the sexes and is approximately 45% to 52% for men and 37% to 48% for women. This is usually measured by spinning down a sample of blood in a test tube, which causes the red blood cells to pack at the bottom of the tube.

Mean cell volume (MCV): The average volume of a red cell. This is a calculated value derived from the hematocrit and red cell count. Normal range is 86 - 98 femtoliters.

Mean corpuscular hemoglobin (MCH): is a calculation of the average amount of oxygen-carrying hemoglobin inside a red blood cell. Macrocytic RBCs are large so tend to have a higher MCH, while microcytic red cells would have a lower value.

Mean Corpuscular Hemoglobin Concentration (MCHC): is the average concentration of hemoglobin in a given volume of red cells. This is a calculated volume derived from the hemoglobin measurement and the hematocrit. Normal range is 32% to 36%.

Red blood cell indices: There are three red blood cell indices: mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). They are measured by a machine and their values come from other measurements in a CBC. The MCV shows the size of the red blood cells. The MCH value is the amount of hemoglobin in an average red blood cell. The MCHC measures the concentration of hemoglobin in an average red blood cell. These numbers help in the diagnosis of different types of anemia. Red cell distribution width (RDW) can also be measured which shows if the cells are all the same or different sizes or shapes.

Platelet count: The number of platelets in a specified volume of blood. Platelets are not complete cells, but actually fragments of cytoplasm (part of a cell without its nucleus or the body of a cell) from a cell found in the bone marrow called a megakaryocyte. Platelets play a vital role in blood clotting. Normal range varies slightly between laboratories but is in the range of 150,000 to 400,000/ cmm (150 to 400 x 109/liter).

Purpose of Complete blood count:
The CBC provides valuable information about the blood and to some extent the bone marrow, which is the blood-forming tissue. The CBC is used for the following purposes:

• to ensure both adequate oxygen carrying capacity and hemostasis
• to see how much blood has been lost if there is bleeding
• to find an infection
• to diagnose anemia
• to identify acute and chronic illness, bleeding tendencies, and white blood cell disorders such as leukemia
• to monitor treatment for anemia and other blood diseases
• to check how the body is dealing with some types of drug or radiation treatment
• to check how abnormal bleeding is affecting the blood cells and counts
• to screen for high and low values before a surgery.

Precautions of complete blood count test:
The CBC requires a sample of blood collected from a vein. The nurse or phlebotomist inserting the needle should clean the skin first. The tourniquet should be removed from the arm as soon as the blood flows. If a fingerstick is used to collect the blood, care must be taken to wipe away the first drop, and not to squeeze the finger excessively as this causes the blood to be diluted by tissue fluid. Many drugs affect the results by causing increased or decreased RBC, WBC, and/or platelet production. Medications should be taken into account when interpreting results.

How to do the complete blood count test (CBC)?
The complete blood count (CBC) test is performed by obtaining a few milliliters (one to two teaspoons) of blood sample directly form the patient. It can be done in many settings including the doctor's office, laboratories, and hospitals. The skin is wiped clean with an alcohol pad, and then a needle is inserted through the area of cleansed skin into to patient's vein (one that can be visualized from the skin.) The blood is then pulled from the needle by a syringe or by a connection to a special vacuumed vial where it is collected. This sample is then taken to the laboratory for analysis. If this blood test is done on a baby, a heel stick will be done instead of a blood draw from a vein.

How it feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks of complete blood count (CBC)
The CBC test is considered a safe procedure. However, as with many medical tests, there are some problems that can occur with having blood drawn:

• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin causing a lump or a bruise)
• pain associated with multiple punctures to locate a vein

Getting the Results
CBC values vary by age and sex. Normal values are ultimately determined by the laboratory performing the test. As a guide, the normal values for men and nonpregnant women are as follows:
WBCs: 4,500–11,000 per microliter for women and men, with neutrophils representing 50–70%, lymphocytes 25–35%, monocytes 4–6%, eosinophils 1–3%, basophils 0.4–1%, and bands 0–5%.
RBCs: 4.2–5.0 million per microliter for women; 4.5–6.2 million per microliter for men.
Hemoglobin: 12–15 g/dL for women; 13.6–17.2 g/dL for men.
Hematocrit: 35–47% for women; 42–52% for men.
Platelets: 150,000 and 350,000 per microliter.
Reticulocyte count: 0.5–1.5%.

Normal adult results for red blood cell indices are as follows:
MCV: 80–98 fl (femtoliters)
MCHC: 32–36%
MCH: 27–31 pg (picograms)
RDW: 11.5–14.5%

In addition to normal values, critical values (alert, panic values) are established for hemoglobin (and hematocrit), WBC count, and platelet count. Precipitously low hemoglobin is associated with hypoxia that can have life-threatening complications. Extremely low WBCs indicates an inability to fight infection and a high risk of sepsis. A severely reduced platelet count predisposes the patient to spontaneous internal bleeding. Representative critical values are shown below:
Hemoglobin: less than 5.0 g/dL
Hematocrit: less than 15%
Platelet count: less than 30,000 per microliter
WBC count: less than 2,500 per microliter and greater than 30,000 per microliter

Abnormal blood count results are seen in a variety of conditions. One of the most common is anemia, which is characterized by a low RBC count, hemoglobin, and hematocrit. The category into which a person's anemia is placed is in part based upon the red blood cell indices provided. The indices provide a significant clue as to the cause of the anemia, but further testing is needed to confirm a specific diagnosis. The most common causes of macrocytic anemia (high MCV) are vitamin B12 and folic acid deficiencies. Lack of iron in the diet, thalassemia (a type of hereditary anemia), and chronic illness are the most common causes of microcytic anemia (low MCV). Normocytic anemia (normal MCV) can be caused by kidney and liver disease, bone marrow disorders, leukemia, excessive bleeding, or hemolysis of the red blood cells. Iron deficiency and thalassemia are the most common causes of hypochromic anemia (low MCHC). Normocytic anemias are usually also normochromic and share the same causes. The red cell distribution width (RDW) is increased in anemias caused by deficiencies of iron, vitamin B12, or folic acid. Abnormal hemoglobins, such as in sickle cell anemia, can change the shape of red blood cells as well as cause them to hemolyze, or rupture. The abnormal shape and the cell fragments resulting from hemolysis increase the RDW. Conditions that cause more immature cells to be released into the bloodstream, such as severe blood loss, will increase the RDW. The larger size of immature cells creates a distinct size variation.

Infections and leukemias are associated with increased numbers of WBCs. Increases or decreases in the percentage of each white cell can be associated with a number of diseases or conditions, including cancer, leukemia, anemia, multiple sclerosis, allergies, parasitic and viral diseases, infections, and tissue damage.