The complete blood count (CBC) remains one of the most common blood tests performed. The CBC includes counts of all three types of blood cells present in a blood sample: red blood cells (RBC), white blood cells (WBC) and platelets (thrombocytes).

The CBC provides important information on the types of blood cells present, and their condition, number and percentage in relation to other cells. The results of a CBC can help diagnose many conditions and diseases. For instance, high WBC levels may indicate a heart attack, infection, an inflammatory disease or leukemia, whereas low levels (leukopenia) can be caused by a wide range of diseases or conditions, including kidney or liver disease, radiation exposure or the presence of a toxic substance in the body.

High RBC levels may indicate a lack of fluid in the body (dehydration), whereas low levels are usually an indicator of anemia. High platelet levels may be due to severe bleeding, infection, strenuous exercise, pregnancy or other factors, whereas low levels (thrombocytopenia) are usually caused by an infection, cancer or a lack of either folic acid or vitamin B-12.

Some medications and dietary supplements may interfere with a CBC. Patients are urged to follow their physician’s orders carefully about how to take their medications before the test in order to ensure its accuracy. There is no need to stop eating or drinking beforehand.

• Obtain a picture of a patient’s overall health status
• Rule out certain diseases or deficiencies
• Estimate fluid levels
• Diagnose a disease, allergy, abnormal condition or inflammation
• Follow and manage a patient’s treatment
• Diagnose infection
• Diagnose diseases of the blood, such as leukemia
• Detect anemia (lack of hemoglobin - a protein in red blood cells)
• Determine the severity of a blood loss
• Monitor the patient’s response to drug or radiation treatment
• Investigate abnormal bleeding or clotting disorders

A CBC may be performed in a hospital or clinic. A nurse or other healthcare professional will draw the blood needed for the test from a vein, usually in the inside of the elbow or the back of the hand. Some patients may experience slight discomfort as the needle is inserted and positioned in the vein.

There is a small risk of dizziness, bruising and infection as a result of the blood withdrawal. Patients who are taking blood-thinning medications (e.g., warfarin, heparin, aspirin) should inform their healthcare professional before the test. These types of medication may increase the risk of excessive bleeding. 

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.

Abnormal levels of one or all parts of a CBC may indicate a variety of conditions, as shown below:

• Red blood cell tests. High levels of RBC, hematocrit or hemoglobin indicate a lack of fluid in the body. This may be caused by smoking, congenital heart disease (cardiac abnormality, defect or malformation that is present from birth), dehydration or kidney disease. Low levels are usually an indicator of anemia, which could be caused by blood loss, malnutrition or leukemia.
  
  
• White blood cell tests. High WBC levels may indicate a heart attack, infection or an inflammatory disease such as rheumatoid arthritis. Severe emotional stress or tissue damage, such as burns, can also cause an elevated WBC. Low levels can be caused by kidney or liver disease, radiation exposure or the presence of a toxic substance in the body. The WBC differential can also help to determine the cause of high WBC levels. For instance, neutrophil increases usually indicate a heart attack or infection, lymphocyte increases can be a sign of leukemia and increased eosinophils can be a sign of an allergic reaction.
  
  
• Platelet tests. Platelet levels may be increased by severe bleeding, infection, strenuous exercise, pregnancy and other factors. Low levels are usually caused by an infection, cancer or a lack of either folic acid or vitamin B-12.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following CBC-related questions:

1. What can the CBC tell you about my health?
   
   
2. Should a CBC be done as a part of my annual physical examination?
   
   
3. Where can I have my blood test performed?
   
   
4. When and from whom can I expect the results?
   
   
5. Can I get a copy of the test results?
   
   
6. What results will indicate an abnormal condition?
   
   
7. What diagnostic tests might follow a CBC?
   
   
8. How should I prepare for my CBC?
   
   
9. Can this test help you diagnose other, unsuspected conditions?
   
   
10. Could the results of the test be influenced by any of the medications I am currently taking?
    
    
11. Will I need regular blood tests for my condition?