complete blood count (CBC)
A complete blood count (CBC), also known as a full blood count (FBC), is a set of medical laboratory tests that provide information about the cells in a person's blood. The CBC indicates the counts of white blood cells, red blood cells and platelets, the concentration of hemoglobin, and the hematocrit (the volume percentage of red blood cells). The red blood cell indices, which indicate the average size and hemoglobin content of red blood cells, are also reported, and a white blood cell differential, which counts the different types of white blood cells, may be included.
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 A CBC specimen in front of a printout displaying CBC and differential results The CBC is often carried out as part of a medical assessment, and can be used to monitor health or diagnose diseases. The results are interpreted by comparing them to reference ranges, which vary with sex and age. Conditions like anemia and thrombocytopenia are defined by abnormal complete blood count results. The red blood cell indices can provide information about the cause of a person's anemia such as iron deficiency and vitamin B12 deficiency, and the results of the white blood cell differential can help to diagnose viral, bacterial and parasitic infections and blood disorders like leukemia. Not all results falling outside of the reference range require medical intervention. The CBC is performed using basic laboratory equipment or an automated hematology analyzer, which counts cells and collects information on their size and structure. The concentration of hemoglobin is measured, and the red blood cell indices are calculated from measurements of red blood cells and hemoglobin. Manual tests can be used to independently confirm abnormal results. Approximately 10–25% of samples require a manual blood smear review,[5] in which the blood is stained and viewed under a microscope to verify that the analyzer results are consistent with the appearance of the cells and to look for abnormalities. The hematocrit can be determined manually by centrifuging the sample and measuring the proportion of red blood cells, and in laboratories without access to automated instruments, blood cells are counted under the microscope using a hemocytometer. In 1852, Karl Vierordt published the first procedure for performing a blood count, which involved spreading a known volume of blood on a microscope slide and counting every cell. The invention of the hemocytometer in 1874 by Louis-Charles Malassez simplified the microscopic analysis of blood cells, and in the late 19th century, Paul Ehrlich and Dmitri Leonidovich Romanowsky developed techniques for staining white and red blood cells that are still used to examine blood smears. Automated methods for measuring hemoglobin were developed in the 1920s, and Maxwell Wintrobe introduced the Wintrobe hematocrit method in 1929, which in turn allowed him to define the red blood cell indices. A landmark in the automation of blood cell counts was the Coulter principle, which was patented by Wallace H. Coulter in 1953. The Coulter principle uses electrical impedance measurements to count blood cells and determine their sizes; it is a technology that remains in use in many automated analyzers. Further research in the 1970s involved the use of optical measurements to count and identify cells, which enabled the automation of the white blood cell differential. PurposeEdit The cells and platelets in human blood. The red blood cells, which carry oxygen are predominant and give rise to the colour of blood. The white blood cells are part of the immune system. The platelets are needed to form clots, which prevent excessive bleeding. Blood is composed of a fluid portion, called plasma, and a cellular portion that contains red blood cells, white blood cells and platelets.[note 1][7] The complete blood count evaluates the three cellular components of blood. Some medical conditions, such as anemia or thrombocytopenia, are defined by marked increases or decreases in blood cell counts.[8] Changes in many organ systems may affect the blood, so CBC results are useful for investigating a wide range of conditions. Because of the amount of information it provides, the complete blood count is one of the most commonly performed medical laboratory tests.[9][10][11] The CBC is often used to screen for diseases as part of a medical assessment.[12] It is also called for when a healthcare provider suspects a person has a disease that affects blood cells, such as an infection, a bleeding disorder, or some cancers. People who have been diagnosed with disorders that may cause abnormal CBC results or who are receiving treatments that can affect blood cell counts may have a regular CBC performed to monitor their health,[4][12] and the test is often performed each day on people who are hospitalized.[13] The results may indicate a need for a blood or platelet transfusion.[14] The complete blood count has specific applications in many medical specialties. It is often performed before a person undergoes surgery to detect anemia, ensure that platelet levels are sufficient, and screen for infection,[15][16] as well as after surgery, so that blood loss can be monitored.[12][17] In emergency medicine, the CBC is used to investigate numerous symptoms, such as fever, abdominal pain, and shortness of breath,[18][19][20] and to assess bleeding and trauma.[21][22] Blood counts are closely monitored in people undergoing chemotherapy or radiation therapy for cancer, because these treatments suppress the production of blood cells in the bone marrow and can produce severely low levels of white blood cells, platelets and hemoglobin.[23] Regular CBCs are necessary for people taking some psychiatric drugs, such as clozapine and carbamazepine, which in rare cases can cause a life-threatening drop in the number of white blood cells (agranulocytosis).[24][25] Because anemia during pregnancy can result in poorer outcomes for the mother and her baby, the complete blood count is a routine part of prenatal care;[26] and in newborn babies, a CBC may be needed to investigate jaundice or to count the number of immature cells in the white blood cell differential, which can be an indicator of sepsis.[27][28] The complete blood count is an essential tool of hematology, which is the study of the cause, prognosis, treatment, and prevention of diseases related to blood.[29] The results of the CBC and smear examination reflect the functioning of the hematopoietic system—the organs and tissues involved in the production and development of blood cells, particularly the bone marrow.[9][30] For example, a low count of all three cell types (pancytopenia) can indicate that blood cell production is being affected by a marrow disorder, and a bone marrow examination can further investigate the cause.[31] Abnormal cells on the blood smear might indicate acute leukemia or lymphoma,[30] while an abnormally high count of neutrophils or lymphocytes, in combination with indicative symptoms and blood smear findings, may raise suspicion of a myeloproliferative disorder or lymphoproliferative disorder. Examination of the CBC results and blood smear can help to distinguish between causes of anemia, such as nutritional deficiencies, bone marrow disorders, acquired hemolytic anemias and inherited conditions like sickle cell anemia and thalassemia.[32][33] The reference ranges for the complete blood count represent the range of results found in 95% of apparently healthy people.[note 2][35] By definition, 5% of results will always fall outside this range, so some abnormal results may reflect natural variation rather than signifying a medical issue.[36] This is particularly likely if such results are only slightly outside the reference range, if they are consistent with previous results, or if there are no other related abnormalities shown by the CBC.[37] When the test is performed on a relatively healthy population, the number of clinically insignificant abnormalities may exceed the number of results that represent disease.[38] For this reason, professional organizations in the United States, United Kingdom and Canada recommend against pre-operative CBC testing for low-risk surgeries in individuals without relevant medical conditions.[15][39][40] Repeated blood draws for hematology testing in hospitalized patients can contribute to hospital-acquired anemia and may result in unnecessary transfusions.[38] Procedure |
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