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What is a Complete Blood Count (CBC)?

Updated on June 10, 2013

The Complete Blood Count, or CBC is one of the most common laboratory procedures done today. It gives the doctor a snapshot of what the general state of the body is and is used in successfully diagnosing a patient. Like its name suggest, the CBC gives information on the general count of the cells in the patient's blood.

The Different Cells in the Blood

Although it may not look like it, our blood is actually composed of millions of tiny cells each with their own specific purpose. There are three main type of cells in the blood:

  • White Blood Cells (WBC) or Leukocytes. These type of cells are the body's main defense against infection.
  • Red Blood Cells (RBC) or Erythrocytes. These cells are what gives the blood its red color. They carry oxygen to the different parts of the body and they carry the waste product Carbon Dioxide to the lungs.
  • Platelets. These cells are responsible for clotting. Every time you get wounded, platelets are responsible for closing that wound up. They also have an effect on general blood thickness.

The Complete Blood Count

The CBC is often used as a screening test. The physician usually requests a CBC lab test to know the general status of the patient. It can also be used to monitor treatments done to the patient or to check for blood problems such as anemia.

The procedure is done by collecting blood through phlebotomy. After blood collection, the blood is brought to the laboratory to be counted manually or through machines known as automated analyzers. A CBC includes:

  • Total RBC count
  • Hematocrit or Packed Cell Volume
  • Hemoglobin
  • RBC Indices
  • Total WBC count
  • WBC differential count
  • Platelet Count

Red Blood Cells
erythrocytosis; dehydration, smoking
anemia, bleeding, bone marrow or kidney problems
often related to RBC count
often related to RBC count
often related to RBC count
often related to RBC count, provides more information in cases of anemia
White Blood Cells
infection, allergic or asthma attack
immune disorders, severe systemic infection, bone marrow disease
increased risk of formation of blood clot in blood vessels
bleeding, some specific diseases such as Dengue Hemorrhagic Fever

RBC Count

The erythrocyte count or the red blood cell count evaluates the amount of red blood cells the patient's blood has. A low RBC count is generally called anemia and can affect the circulation of oxygen in the body causing the symptoms of anemia (weakness, paleness, shortness of breath, etc.). On the other hand, a high RBC count is called erythrocytosis. It is less serious than anemia and is usually caused by smoking or dehydration, although rarely, points to more serious complications such as bone marrow or kidney diseases.

Hematocrit and Hemoglobin

Hematocrit is the percentage of red blood cells in an amount of blood. The result is expressed in percentages, usually in decimal form meaning a result of 0.45 means that 45% of the blood is composed of red blood cells. The results of Hematocrit is often associated with the results of the total RBC count. Lowered Hematocrit is usually observed with bleeding patients.

Hemoglobin is a part of the red blood cell and it's responsible for carrying oxygen and carbon dioxide throughout the body. It also gives RBCs its red color. The hemoglobin count measures the oxygen carrying capacity of the RBC. A normal total RBC count and a low hemoglobin count means that although the body produces enough red blood cells, most of these cells cannot carry oxygen.

RBC Indices

These tests are used to determine the shape, color and hemoglobin content of the blood. They are usually computed from the other three RBC counts and abnormal results often point to a diagnosis of the type of anemia present. This test includes:

  • Mean Cell Volume (MCV). This measure the average RBC size. A high value can represent macrocytic anemia while a low value can represent microcytic anemia. Normal MCV means the cells are normocytic.
  • Mean Cell Hemoglobin (MCH). This measures the amount of hemoglobin in an average RBC
  • Mean Cell Hemoglobin Concentration (MCHC). This measures the average concentration in an RBC. This also reflects the average color of the red blood cell. High values mean the RBCs are hyperchromic while low values mean they are hypochromic. A normal value is called normochromic.

An example of WBCs on stained blood smear. The tiny specs are the platelets.
An example of WBCs on stained blood smear. The tiny specs are the platelets. | Source

WBC Count

The WBC count shows the amount of Leukocytes in the blood. WBC is the body's defense against infection. A high amount of WBC (called leuckocytosis) can mean that the body is currently fighting an infection, although it can also mean that there is inflammation or allergy present. In rare cases, an unusually high WBC count can also lead to the diagnosis of leukemia. Low WBC counts, known as leukopenia, are rare than elevated WBCs and is seen in serious conditions such as bone marrow diseases, immune system diseases and extremely severe infections.

WBC Differential Count

There are 5 major types of white blood cell each with a specific function in protecting the body. An elevation of one specific type can shine some light into what type of infection is currently taking place in the body.

  • Neutrophils (also called Segmenters). These are the most abundant type of WBC. An elevated count of neutrophils can be seen in bacterial infections although it can also be elevated in acute viral infections
  • Lymphocyte. A high amount of lymphocytes can be seen in viral infections although it can also be seen in some specific bacterial infections such as tuberculosis.
  • Monocyte. Usually low, with a normal differential count often reporting 1 or 2 only. An increased amount of monocyte could indicated chronic infection.
  • Eosinophil. Usually low count, Eosinophils increase in parasitic infections, allergies or asthma attacks.
  • Basophils. Rarely seen in a regular differential count. An abnormally high amount could be bone marrow releated.

bacterial infections
bone marrow problems
viral infections
bone marrow problems, immune disorders
chronic infections
parasitic infections, allergic or asthma attacks
bone marrow damage

Platelet Count

Platelets are responsible for the clotting in the blood. A low platelet count can lead to hemorrhage or uncontrolled bleeding while a high count can mean that there is an increased chance of a blood clot forming in a vein or artery. In certain diseases such as Dengue Hemorrhagic Fever, the platelet count is used to monitor the effectivity of treatment.

In conclusion...

There are many other things that can affect a Complete Blood Count. Always go to a physician first especially when medication is involved. What is written here is meant as a general guideline and is not meant to replace a physician's opinion or diagnosis.

If you're preparing for a CBC test, have sufficient rest and a full meal. Always stay hydrated. You can bring bottled water to the laboratory so you can drink while waiting. Being tired, hungry or dehydrated can affect the results of the test and can lead to a misdiagnosis!


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