Complete Blood Count

Does this test have other names?    


What is this test?

The complete blood count (CBC) is a blood test used to screen your overall health and to look for many different illnesses, including anemia, infections, and leukemia. The test extracts a large amount of information from the blood sample you've given, including:

  • The number and types of white blood cells (WBCs). There are five types of WBCs, which all play a role in fighting infection. High numbers of WBCs, or of a specific type of WBC,  may mean you have an infection or inflammation somewhere in your body, among other things. Low numbers of WBCs may mean you are at risk for infections.

  • The number of red blood cells (RBCs). RBCs carry oxygen throughout the body and remove excess carbon dioxide. Too few RBCs may be a sign of different conditions or diseases. In rare cases, too many RBCs may cause problems with blood flow.

  • Variation in the size of your red blood cells. This test is known as red cell distribution width (RDW).You'll probably have greater differences in red blood cell size, for example, if you have anemia.

  • The percentage of red blood cells in a certain volume of whole blood (hematocrit). A low hematocrit may be a sign of excessive bleeding or of a problem making new red blood cells, such as because of an iron deficiency. A higher than normal hematocrit can stem from dehydration or other disorders.

  • A hemoglobin count. This protein in red blood cells carries oxygen from the lungs to the rest of the body. Abnormalities can be a sign of problems ranging from anemia to lung disease.

  • The average size of your red blood cells. This test is known as mean corpuscular volume (MCV). MCV goes up when your red blood cells are bigger than normal, which may be seen with anemia caused by low vitamin B12 or folate levels. If your red blood cells are smaller, this can indicate other types of anemia, such as from iron deficiency.

  • A platelet count. Platelets are cell fragments that play a role in blood clotting. Too few platelets may signal an increased risk of bleeding, and too many may mean a number of possible conditions. 

Why do I need this test?

You may need this test if you have:

  • Unusual bleeding or bruising

  • Infection or inflammation

  • Persistent weakness and fatigue that your doctor suspects may be symptoms of anemia

You may also have this test if your doctor suspects you may have a certain medical disease or condition. Or you may receive it as part of a routine exam to check your health status. In addition, the test may be used to monitor the effectiveness of certain treatments.

What other tests might I have along with this test?

Your doctor may order additional tests to confirm a diagnosis or to figure out the most effective therapy for you. For example, you may have a bone marrow test if your CBC shows that you may have a bone marrow disease. If your CBC shows that you have anemia, your doctor may order tests to look for possible causes, such as checking your iron, folate, and vitamin B12 levels. 

What do my test results mean?

Many things may affect your lab test results, including the method each lab uses to do the test. Even if your test results are different from the normal values, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Although estimates vary from lab to lab, here are some typical normal ranges for the main parts of the CBC:

  • Red blood cell count: 3.93 to 5.69 million cells per cubic millimeter

  • Hemoglobin: 12.6 to 17.5 grams per deciliter (g/dL) for males; 12.0 to 16 g/dL for females

  • Hematocrit: 38 to 47.7  percent

  • White blood cell count: 3,300 to  8,700 cells per cubic millimeter

  • Platelet count: 147,000 to 347,000 per cubic millimeter

Abnormal test results can have many causes, some of which might not mean that you have a problem that requires treatment. The most common abnormality found through the CBC is mild anemia. You may have further testing, depending on how severe the anemia is and whether other abnormalities crop up in the test.   

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Certain medications might affect your results, so talk with your doctor about the medicines you are taking.

How do I get ready for this test?

You don't need to prepare for this test. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use