A comprehensive blood count---more commonly known as a complete blood count, or CBC---is a blood test used to determine the amounts of each specific type of blood cell present in circulating blood. A low blood count means that one or more of the blood cell types is present in lower numbers than usual. This decrease is often caused by an underlying medical condition or problem.
A complete blood count determines the number of four separate types of blood cells or proteins in circulating blood: red blood cells, white blood cells, platelets, and haemoglobin. The test also determines the exact proportion of the blood's red blood cell to plasma ratio, a number that is known as the hematocrit. A decreased population in the blood of any of the four blood cells or proteins can be caused by a variety of disorders, and gives physicians a starting point for further diagnosis and treatment.
Low red blood cell count
A low red blood cell count is most often caused by conditions that disrupt the body's ability to produce properly functioning red blood cells. These conditions can involve a lack of iron, as in iron deficiency anaemia, or can be caused by disorders that suppress the bone marrow's ability to produce the cells, such as kidney disease, pernicious anaemia or aplastic anaemia. Pregnancy, excessive blood loss from cancer or internal ulcers, and genetic disorders such as sickle cell anaemia also cause red blood cell counts to drop.
Low white blood cell count
A low white blood cell count can be the result of bone marrow damaging disorders such as cancer; immune diseases, such as HIV, AIDS or rheumatoid arthritis that target and destroy white blood cells; or drugs and medications that damage the bone marrow's blood cell producing capabilities. Low white blood cells counts can also be the result of inheritable disorders, but this is much less common.
Low platelet count
A low platelet count in the blood---a condition known as thrombocytopenia---is usually caused by either a severe decrease in the amount of platelets being produced by the bone marrow or a medical disorder that involves the destruction or abnormally rapid use of the platelets. Conditions that can result in a low platelet count include severe bacterial or viral infections, autoimmune diseases, certain types of cancer and genetic disorders.
Low haemoglobin count
Haemoglobin is the protein in red blood cells that is responsible for carrying oxygen. Haemoglobin and red blood cell counts go hand-in-hand: if one is low, it is indicative of a decrease in the other. Since haemoglobin is dependent on iron to function properly, iron deficiencies in the body can contribute to a low haemoglobin count. Other low haemoglobin count causes are similar to those of low red blood cell count causes---blood loss, autoimmune disorders, medications that disrupt the function of the bone marrow and disorders such as cancer or kidney disease.