The complete blood count (CBC) is a very common blood test. It evaluates the three major types of cells in blood: red blood cells, white blood cells, and platelets.
Doctors often order a CBC for a child to check for anemia, infections, or other health problems.
How Is a CBC Test Taken?
It doesn't take much blood to perform a CBC. A health professional may take it by sticking a child's heel or finger with a small, sharp surgical instrument. The blood for a CBC also can be taken from a vein. After the skin surface is cleaned, a needle is inserted into a vein (usually in the arm) and blood is withdrawn.
Either method of collecting a sample is only temporarily uncomfortable and can feel like a gentle pinprick. The sample is collected in special tubes and then processed by a machine, usually referred to as a hematology analyzer.
CBC results can be available in minutes in an emergency, but more commonly come after a few hours or the next day.
Parts of the CBC
Red Blood Cell Count, Hemoglobin, and MCV
Three tests - measuring red blood cell (RBC) count, hemoglobin, and mean (red) cell volume (MCV) - provide information about the red blood cells, which carry oxygen from the lungs to the rest of the body. These tests are usually done to test for anemia, a common condition that occurs when there aren't enough red blood cells.
- The red blood cell count is a measure of the number of RBCs in the body.
- Hemoglobin is the oxygen-carrying protein in red blood cells. RBCs carry oxygen to all parts of the body.
- MCV measures the average size of the red blood cells.
Other factors analyzed include the hematocrit (HCT), which is the percentage of red blood cells in the blood sample. If a child has anemia, the results for RBC, hemoglobin, and hematocrit will all be low.
Many things can cause anemia. Sometimes it occurs because not enough RBCs are being made and other times it's because the cells are either being destroyed or lost through bleeding. If a CBC points to anemia, your child's doctor will likely order other lab tests to determine what's causing it and how to treat it.
White Blood Cell Differential Count
Also part of the CBC is the blood differential test that measures the relative numbers of white blood cells (WBCs) in the blood. WBCs (also called leukocytes) help the body fight infection. These cells are bigger than red blood cells, and there are far fewer of them in the bloodstream. An abnormal white blood cell count may indicate that there is an infection, inflammation, or other stress in the body. For example, a bacterial infection can cause the WBC count to increase or decrease dramatically.
There are five types of white blood cells: neutrophils, lymphocytes, eosinophils, basophils, and monocytes. Each has a different job.
The two major types of WBCs are neutrophils and lymphocytes. Neutrophils play a key role in the body's defense against invading bacteria by destroying invading organisms. Someone with insufficient neutrophils is at risk for developing serious infections. Lymphocytes produce antibodies, specific proteins that attack and help destroy specific germs. They are especially important in fighting viral infections, like colds and flu. People with advanced HIV can have low lymphocyte counts, increasing their risk for developing certain infections.
Eosinophils and basophils in the blood may be increased in allergic conditions. Monocytes, the largest white blood cells in the bloodstream, remove dead cells and organisms from the blood.
The Platelet Count
Platelets are the smallest blood cells. They play an important role in blood clotting and the prevention of bleeding. When a blood vessel is damaged or cut, platelets clump together and plug up the hole until the blood clots. If the platelet count is too low, a person can be in danger of bleeding in any part of the body.