What Does a CBC Measure?
A CBC measures the following:
- The number of red blood cells (RBCs) The primary function of a RBC is to maintain a normal concentration of hemoglobin.
- The number of white blood cells (WBCs) There are five types of WBCs: neutrophils, eosinophils, basophils, monocytes and lymphocytes. Each has its own function in the fight against infection
- Hemoglobin The major component of the RBC, hemoglobin enables the RBC to carry oxygen from the lungs to the tissues and then carry carbon dioxide from the tissues back to the lungs for excretion
- Hematocrit Measures the percentage of packed RBCs in whole blood
- Mean corpuscular volume (MCV) Expresses the average size of RBCs and indicates whether or not they are oversized, undersized or normal.
- The platelet count The smallest formed elements in the blood, platelets promote coagulation (clots after injury).
How the Test Is Performed
The test is performed by drawing blood from a vein, usually from the arm or hand. An elastic band is placed above the puncture site which is cleaned thoroughly with an antiseptic wipe, such as alcohol. After the site is cleansed, a needle is inserted into the vein and the blood is collected into an airtight vial. The band is then released, circulation restored to the area and a pressure bandage placed onto the puncture site. Extra caution should be used if drawing blood from a patient who is on anticoagulants. In this case, direct pressure should be applied for a short period of time to stop excess bleeding from the puncture site.
How to Prepare for the Test
No preparation is needed for a CBC.
Source:
Illustrated Guide to Diagnostic Tests. Springhouse Corporation. 1998.
