Complete blood cell count
A complete blood count (CBC), also known as full blood count (FBC) or full blood exam (FBE) or blood panel, is a test panel requested by a doctor or other medical professional that gives information about the cells in a patient's blood. A scientist or lab technician performs the requested testing and provides the requesting medical professional with the results of the CBC.
Blood counts of various types have been used for clinical purposes since the 19th century. Automated equipment to carry out complete blood counts was developed in the 1950s and 1960s.
The cells that circulate in the bloodstream are generally divided into three types: white blood cells (leukocytes), red blood cells (erythrocytes), and platelets (thrombocytes). Abnormally high or low counts may indicate the presence of many forms of disease, and hence blood counts are amongst the most commonly performed blood tests in medicine, as they can provide an overview of a patient's general health status. A CBC is routinely performed during annual physical examinations in some jurisdictions.
File:CBC blood test.ogv A phlebotomist collects the sample, drawing the blood into a test tube containing an anticoagulant (EDTA, sometimes citrate) to stop it from clotting. The sample is then transported to a laboratory. Sometimes the sample is drawn off a finger prick using a Pasteur pipette for immediate processing by an automated counter.
In the past, counting the cells in a patient's blood was performed manually, by viewing a slide prepared with a sample of the patient's blood under a microscope (a blood film, or peripheral smear). Presently, this process is generally automated by use of an automated analyzer, with only approximately 10–20% of samples now being examined manually.
Automated blood count
The blood is well mixed (though not shaken) and placed on a rack in the analyzer. This instrument has flow cells, photometers and apertures that analyze different elements in the blood. The cell counting component counts the numbers and types of different cells within the blood. The results are printed out or sent to a computer for review.
Blood counting machines aspirate a very small amount of the specimen through narrow tubing followed by an aperture and a laser flow cell. Laser eye sensors count the number of cells passing through the aperature, and can identify them; this is flow cytometry. The two main sensors used are light detectors and electrical impedance. The instrument measures the type of blood cell by analyzing data about the size and aspects of light as they pass through the cells (called front and side scatter). Other instruments measure different characteristics of the cells to categorize them.
Because an automated cell counter samples and counts so many cells, the results are very precise. However, certain abnormal cells in the blood may not be identified correctly, requiring manual review of the instrument's results and identification of any abnormal cells the instrument could not categorize.
In addition to counting, measuring and analyzing red blood cells, white blood cells and platelets, automated hematology analyzers also measure the amount of hemoglobin in the blood and within each red blood cell. This is done by adding a diluent that lyses the cells which is then pumped into a spectro-photometric measuring cuvette. The change in color of the lysate equates to the hemoglobin content of the blood. This information can be very helpful to a physician who, for example, is trying to identify the cause of a patient's anemia. If the red cells are smaller or larger than normal, or if there is a lot of variation in the size of the red cells, this data can help guide the direction of further testing and expedite the diagnostic process so patients can get the treatment they need quickly.
Manual blood count
Counting chambers that hold a specified volume of diluted blood (as there are far too many cells if it is not diluted) are used to calculate the number of red and white cells per litre of blood.
To identify the numbers of different white cells, a blood film is made, and a large number of white cells (at least 100) are counted. This gives the percentage of cells that are of each type. By multiplying the percentage with the total number of white blood cells, the absolute number of each type of white cell can be obtained.
Manual counting is useful in cases where automated analyzers cannot reliably count abnormal cells, such as those cells that are not present in normal patients and are only seen in peripheral blood with certain haematological conditions. Manual counting is subject to sampling error because so few cells are counted compared with automated analysis.
Medical technologists examine blood film via a microscope for some CBCs, not only to find abnormal white cells, but also because variation in the shape of red cells is an important diagnostic tool. Although automated analysers give fast, reliable results regarding the number, average size, and variation in size of red blood cells, they do not detect cells' shapes. Also, some normal patients' platelets will clump in EDTA anticoagulated blood, which causes automatic analyses to give a falsely low platelet count. The person viewing the slide in these cases will see clumps of platelets and can estimate if there are low, normal, or high numbers of platelets.
A complete blood count will normally include:
- Total white blood cells — All the white cell types are given as a percentage and as an absolute number per litre.
- Total red blood cells — The number of red cells is given as an absolute number per litre. (Iron deficiency Anemia shows up as a Low RBC count.)
- Hemoglobin – The amount of hemoglobin in the blood, expressed in grams per decilitre. (A low level of Hemoglobin is a sign of anemia.)
- Template:SearchLOINC(Note: not all the codes in the search result are suitable for this test.)
- Hematocrit or packed cell volume (PCV) – This is the fraction of whole blood volume that consists of red blood cells.
- Red blood cell indices
- Mean corpuscular volume (MCV) – the average volume of the red cells, measured in femtolitres. Anemia is classified as microcytic or macrocytic based on whether this value is above or below the expected normal range. Other conditions that can affect MCV include thalassemia, reticulocytosis, alcoholism, chemotherapy, vitamin B12, and/or Folic acid deficiency.
- Mean corpuscular hemoglobin (MCH) – the average amount of hemoglobin per red blood cell, in picograms.
- Mean corpuscular hemoglobin concentration (MCHC) – the average concentration of hemoglobin in the cells.
- Red blood cell distribution width (RDW) – the variation in cellular volume of the RBC population.
A complete blood count with differential/platelet will also include:
- Neutrophil granulocytes — May indicate bacterial infection. May also be raised in acute viral infections. Because of the segmented appearance of the nucleus, neutrophils are sometimes referred to as "segs." The nucleus of less mature neutrophils is not segmented, but has a band or rod-like shape. Less mature neutrophils — those that have recently been released from the bone marrow into the bloodstream — are known as "bands" or "stabs". Stab is a German term for rod.
- Lymphocytes — Higher with some viral infections such as glandular fever and. Also raised in chronic lymphocytic leukemia (CLL). Can be decreased by HIV infection. In adults, lymphocytes are the second most common WBC type after neutrophils. In young children under age 8, lymphocytes are more common than neutrophils.
- Monocytes — May be raised in bacterial infection, tuberculosis, malaria, Rocky Mountain spotted fever, monocytic leukemia, chronic ulcerative colitis and regional enteritis 
- Eosinophil granulocytes — Increased in parasitic infections, asthma, or allergic reaction.
- Basophil granulocytes — May be increased in bone marrow related conditions such as leukemia or lymphoma.
A manual count will also give information about other cells that are not normally present in peripheral blood, but may be released in certain disease processes.
- Platelet numbers are given, as well as information about their size and the range of sizes in the blood.
- Mean platelet volume (MPV) – a measurement of the average size of platelets.
An example report format for a complete blood count. Note that test names, measurement units and reference ranges may vary between countries and laboratories. Patient results should always be interpreted using the units and reference ranges from the laboratory that produced the results.
Certain disease states are defined by an absolute increase or decrease in the number of a particular type of cell in the bloodstream. For example:
|Type of Cell||Increase||Decrease|
|Red Blood Cells (RBC)||erythrocytosis or polycythemia||anemia or erythroblastopenia|
|White Blood Cells (WBC):||leukocytosis||leukopenia|
|– lymphocytes||– lymphocytosis||– lymphocytopenia|
|– granulocytes:||– granulocytosis||– granulocytopenia or agranulocytosis|
|– –neutrophils||– –neutrophilia||– –neutropenia|
|– –eosinophils||– –eosinophilia||– –eosinopenia|
|– –basophils||– –basophilia||– –basopenia|
|All cell lines||–||pancytopenia|
Many disease states are heralded by changes in the blood count:
- Leukocytosis can be a sign of infection
- Thrombocytopenia can result from drug toxicity
- Pancytopenia is generally referred to as the result of decreased production from the bone marrow, and is a common complication of cancer chemotherapy
- NCBI Bookshelf ID: NBK2261
- Complete Blood Count - Lab Tests Online
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