Laboratory Tests for Blood Disorders

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 (blood smear).

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 cells suggest anemia due to a deficiency of folatedeficiency of vitamin B12.

White blood cell parameters evaluated by the CBC include the

• Total number of white blood cells

• Percentages and numbers of the different types of white blood cells

The white blood cells are the major component of the body's immune system. There are normally five major types of white blood cells (neutrophils, lymphocytes, monocytes, eosinophils, and basophils), and different types are recruited into service when the immune system responds to different stresses or disorders. Counting the number of white blood cells of each type (differential white blood cell count) can suggest to a doctor possible causes of a change in the total white blood cell count. For example, if a person with diarrhea who recently travelled to certain areas has an increased white blood cell count due to increased eosinophils, the doctor would likely suspect a parasitic infection because eosinophils are more often recruited to fight parasitic 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).

Platelets are also counted as part of a CBC. Platelets are cells that help in the clotting process by gathering at a bleeding site and clumping together to form a plug. 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). Too few platelets may impair blood clotting. A high number of platelets (thrombocytosis) can lead to excessive blood clotting in small blood vessels, especially those in the heart or brain. However, in some disorders, a high number of platelets may paradoxically result in excess bleeding.

Although automated machines can rapidly determine the number of different blood cells and provide information about the size and shape of red blood cells and types of white blood cells, examining a blood sample under a microscope may provide additional information. To do this, a drop of blood is smeared across a glass slide to form a thin layer that makes it easy to see individual blood cells. The slide is then stained with colored chemicals to reveal specific characteristics of the blood cells and examined under the microscope. An experienced examiner can often obtain better information about cell counts, size and shape, and specific characteristics of the cells than a machine can.

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 0.5 to 2.5% of the total number of red blood cells. When the body needs more red blood cells, as after blood loss, 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 some 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.

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 some of these proteins to look for abnormalities in their quantity or structure that occur in certain blood disorders. For example, in multiple myeloma, certain bone marrow cells, called plasma cells, become cancerous and produce unusual antibody (immunoglobulin) proteins (including Bence Jones proteins) that can be measured in blood and in 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 (for example, vitamin B12 and folate) that are necessary for the production of healthy blood cells also can be measured.

Blood type is determined by the presence of certain proteins on the surface of red blood cells. Blood typing must be done before blood can be transfused.