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Laboratory Tests for Blood Disorders
Doctors select tests to help diagnose blood disorders based on the person's symptoms and the results of the physical examination (see Medical History and Physical Examination for Blood Disorders). Sometimes a blood disorder causes no symptoms but is discovered when a laboratory test is done for another reason. For example, a complete blood count done as part of a regular check-up may reveal anemia. When a blood disorder is suspected, a complete blood count and other tests may need to be done to determine the specific diagnosis.
The blood test most commonly done is the complete blood count (CBC). The CBC is an evaluation of all the cellular components (red blood cells, white blood cells, and platelets) in the blood. Automated machines do this test in less than 1 minute on a small amount of blood. The CBC is supplemented in some instances by examination of blood cells under a microscope.
Red blood cell parameters evaluated by CBC include
Number of red blood cells (red blood cell count, RBCs)
Proportion of blood made up of red blood cells (hematocrit, Hct)
Amount of hemoglobin (the oxygen-carrying protein in red blood cells) in the blood (hemoglobin, Hb)
Average size of red blood cells (mean cellular volume, MCV)
Variability of size of red blood cells (red cell distribution width, RDW)
Amount of hemoglobin in an individual red blood cell (mean cellular hemoglobin, MCH)
Concentration of hemoglobin in an individual red blood cell (mean cellular hemoglobin concentration, MCHC)
Abnormalities in these parameters can alert laboratory workers to the presence of abnormalities in the red blood cells (which may then be further evaluated by examination under a microscope).
Abnormal red blood cells may be fragmented or shaped like teardrops, crescents (sickle-shaped), or a variety of other forms. Knowing the specific shape and size of red blood cells can help a doctor diagnose a particular cause of anemia. For example, sickle-shaped cells are characteristic of sickle cell disease, small cells containing insufficient amounts of hemoglobin are likely due to iron deficiency anemia, and large oval cells suggest anemia due to a deficiency of folate (folic acid) or vitamin B12.
The CBC also provides information about the white blood cells (see Components of Blood : White Blood Cells), which are the major component of the body's immune system (see Overview of the Immune System).
White blood cellparameters evaluated by the CBC include the
There are several types of white blood cells, and different types are recruited into service when the immune system responds to different disorders. Counting the number of white blood cells of each type (differential and absolute white blood cell counts) can suggest to a doctor possible causes of a change in the total white blood cell count. For example, if a person with cold symptoms has an increased white blood cell count due to increased neutrophils, the doctor would likely suspect a bacterial pneumonia rather than a viral infection because neutrophils are more often recruited to fight bacterial infections.
To provide more information about the white blood cells, the doctor can examine these cells under a microscope. The microscopic examination can identify features of the cells that are characteristic of certain diseases. For example, large numbers of white blood cells that have a very immature appearance (blasts) may indicate leukemia (cancer of the white blood cells—see Overview of Leukemia).
Platelets are usually also counted as part of a CBC. The number of platelets is an important measure of the blood's ability to form blood clots (forming blood clots is the body's protective mechanism for stopping bleeding). A high number of platelets (thrombocytosis) can lead to blood clots in small blood vessels, especially those in the heart or brain. In some disorders, a high number of platelets may paradoxically result in excess bleeding.
Complete Blood Count (CBC)
The reticulocyte count measures the number of newly formed (young) red blood cells (reticulocytes) in a specified volume of blood. Reticulocytes normally make up about 1% of the total number of red blood cells. When the body needs more red blood cells, as in anemia, the bone marrow normally responds by producing more reticulocytes. Thus, the reticulocyte count is a measure of the capacity of the bone marrow to make new red blood cells.
Once a doctor determines that something is wrong with one or more of the cell types in the blood, many additional tests are available to shed more light on the problem. Doctors can measure the proportion of the different types of white blood cells and can determine subtypes of these cells by assessing certain markers on the surface of the cells. Tests are available to measure the ability of white blood cells to fight infection, to assess the functioning of platelets and their ability to clot, and to measure the contents of red blood cells to help determine the cause of anemia or why the cells are not functioning properly. Most of these tests are done on samples of blood, but some require a sample from the bone marrow (see Bone Marrow Examination).
One measure of the body's ability to stop bleeding is the count of the number of platelets. Sometimes doctors need to test how well the platelets function. Other tests can measure the overall function of the many proteins needed for normal blood clotting (clotting factors). The most common of these tests are the prothrombin time (PT) and the partial thromboplastin time (PTT). The levels of individual clotting factors can also be determined.
Blood plasma (the liquid portion of blood) contains many proteins. Urine contains very small amounts of protein. Doctors sometimes measure these proteins to look for abnormalities in their quantity or structure. For example, in multiple myeloma, certain bone marrow cells, called plasma cells, become cancerous and produce unusual antibody proteins (including Bence Jones proteins) that can be measured in blood and urine.
Erythropoietin is a protein made in the kidneys that stimulates the bone marrow to produce red blood cells. The level of this protein can be measured in the blood. Levels of iron and certain vitamins that are necessary for the production of healthy blood cells also can be measured.
Blood type, which is determined by the presence of certain proteins on the surface of red blood cells, can be identified by measuring the reaction of a small sample of a person's blood to certain antibodies. Blood typing requires evaluation of both the plasma and red blood cells. Blood typing must be done before blood can be transfused (see Blood typing).
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