The CBC includes the following tests:

• Red blood cell tests
• • Red blood cell count
  • Hematocrit
  • Hemoglobin
    
    
• White blood cell tests
• • White blood cell count
  • Differential count
    
    
• Platelet tests
• • Platelet count

Red blood cell tests

A major test of the red blood cells is the red blood cell (RBC or erythrocyte) count. This test counts the number of red blood cells within a specific amount (usually a cubic millimeter) of blood. The normal range may vary slightly from lab to lab but it usually falls between 4.2 and 6.9 million cells per cubic millimeter of blood. Babies and people who live at high altitudes will have slightly higher counts.

Hematocrit (Hct) refers to the percentage of blood consisting of red blood cells. Hematocrit (meaning “to separate blood”) is an indicator of both the number and the size of red blood cells. A hematocrit of 45 would mean that red blood cells compose 45 percent of a blood sample. The normal range for hematocrit varies slightly between the sexes with a range of 45 to 62 percent for men and 37 to 48 percent for women.

Hemoglobin (Hgb or Hb) is the protein molecule in red blood cells that carries oxygen as blood circulates throughout the body. Hemoglobin gives blood its red color. The hemoglobin test measures the amount of hemoglobin in a patient’s blood and gives a good indication of a blood cell’s ability to transport oxygen. The normal range for hemoglobin varies slightly between the sexes, with a normal value of 13 to 18 grams per deciliter for men and 12 to 16 grams per deciliter for women.

Other items that may be included on a CBC test are:

• Mean corpuscular volume (MCV). The average size of a red blood cell.
  
  
• Mean corpuscular hemoglobin (MCH). The average amount of hemoglobin per red blood cell.
  
  
• Mean corpuscular hemoglobin concentration (MCHC). The average concentration of hemoglobin per red blood cell
  
  
• Red cell distribution width (RDW). A calculation of the variation in size of red blood cells.

White blood cell tests

A major test of the white blood cells is the white blood cell (WBC or leukocyte) count. This test counts the number of white blood cells within a specific amount (usually a cubic millimeter) of blood. White blood cells protect the body from infection by attacking invading bacteria, viruses and other foreign materials in the body. Though usually fewer in number than red blood cells, the number of white blood cells rises dramatically when infection is present. Therefore, a WBC is commonly used to indicate the presence of an infection such as appendicitis, a blood disease such as leukemia or to monitor the body’s response to cancer treatment. The normal range for this test varies slightly from lab to lab but usually falls within 4,300 and 10,800 white blood cells per cubic millimeter of blood.

In addition to counting the total number of white blood cells, a differential count may also be done to count the different types of white blood cells present in a blood sample. There are five major types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils and basophils. Expressed as a percentage of the white blood cells examined, the normal range for the different types of white blood cells in a blood sample is as follows:

• Neutrophils (47 to 77 percent). Also known as polys, PMNS or granulocytes, neutrophils are the most common type of white blood cell in circulation. Their numbers become elevated when infection is present. A subtype of neutrophils are band neutrophils. These are immature forms of neutrophils that represent only 0 to 3 percent of the white blood cells in peripheral blood. When their number increases, it can signal an infection such as appendicitis.
  
  
• Lymphocytes (16 to 43 percent) are found to be elevated in some types of leukemia.
  
  
• Monocytes (.5 to 10 percent) are found to be elevated in different kinds of leukemia.
  
  
• Eosinophils (.3 to 7 percent) are found to be elevated in many allergic states.
  
  
• Basophils (.3 to 2 percent) can be elevated in different types of blood disease and poisonings.

Each of these different types of white blood cells plays a different role in protecting the body. Examining and counting the percentage of each type of white cell present can provide important information about the patient’s immune system. The maturity of the white cells or an abnormal number of various types of white blood cells can provide valuable information about infection, allergies, reactions to drugs, radiation or the presence of blood diseases such as leukemia.

Platelet tests

A major test of the platelets is the platelet (thrombocyte) count. This test counts the number of platelets within a specific amount (usually a cubic millimeter) of blood. Platelets are the smallest type of blood cell and play a major role in blood clotting. Too few platelets can lead to uncontrolled bleeding. Too many platelets can lead to the formation of a potentially dangerous blood clot in a blood vessel. Platelet counts are also used to diagnose a wide variety of disorders, including genetic disorders, lupus, pernicious anemia, leukemia and to monitor the effects of chemotherapy. Normal range values vary slightly between laboratories but usually fall between 150,000 and 350,000 per cubic millimeter.