A complete blood count (CBC) is one of the most common tests performed in a medical laboratory. It provides important information about the type, number and other characteristics of cells found in the blood: red blood cells (RBCs), white blood cells (WBCs) and platelets. RBCs transport oxygen to tissues, and an abnormality in the number, size or hemoglobin content of these cells may indicate a deficiency (anemia) or, more rarely, an excess of hemoglobin (e.g., dehydration, polycythemia). WGBs, or leukocytes, include several types of cells (neutrophils, monocytes, etc.) that are involved in the body’s defences against infections of all kinds, including allergic reactions. Platelets are used for blood coagulation, therefore a platelet deficiency may cause abnormal bleeding while an excessive count will promote the formation of abnormal clots (thrombosis, phlebitis). RBCs, WBCs and platelets are all formed in bone marrow. Therefore, a CBC provides important information on the proper functioning of bone marrow (response to infections or anemia, production abnormalities due to poisons or abnormal production of certain categories of cells (leukemia, etc.). A CBC will also reveal abnormalities affecting other elements essential for the production and proper functioning of blood cells (functioning of the spleen, liver and kidneys; nutritional status of amino acids, iron, vitamin B12, folic acid, etc.).