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Laboratory Tests for Blood Disorders


David J. Kuter

, MD, DPhil, Harvard Medical School

Reviewed/Revised Sep 2023
Topic Resources

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 a low red blood cell count (anemia Overview of Anemia Anemia is a condition in which the number of red blood cells is low. Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts... read more ). When a blood disorder is suspected, a complete blood count and other tests may need to be done to determine the specific diagnosis.

How Blood Is Obtained

Blood is obtained from a vein with a needle to fill one or more sample tubes or sometimes from the fingertip by a needle prick to get just a drop of blood.

A health care practitioner determines which vein to use, usually one on the inside surface of the person's elbow. A tourniquet is applied around the upper arm, causing the veins below it to fill with blood so that they may be more easily seen or felt.

After the skin immediately surrounding the vein is cleaned thoroughly, a needle is inserted into the vein. A stinging sensation is usually felt when the needle is first inserted, but otherwise the procedure is painless.

Blood moves through the needle and into a syringe or collecting tube. Once enough blood is collected, the tourniquet is removed, the needle is then removed from the vein, and pressure is applied to the area to prevent bleeding from the puncture site.

If only a small amount of blood is needed, the area, usually a finger (the heel in infants), is cleaned and a needle is used to prick the skin.

Complete blood count

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 Red Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more Red Blood Cells , white blood cells White Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more White Blood Cells , and platelets Platelets The main components of blood include Plasma Red blood cells White blood cells Platelets read more 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).

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 White Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more White Blood Cells are the major component of the body's immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more . 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 Overview of Leukemia Leukemias are cancers of white blood cells or of cells that develop into white blood cells. White blood cells develop from stem cells in the bone marrow. Sometimes the development goes awry... read more (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 How Blood Clots Hemostasis is the body's way of stopping injured blood vessels from bleeding. Hemostasis includes clotting of the blood. Too little clotting can cause excessive bleeding from minor injury Too... read more (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 Essential Thrombocythemia Essential thrombocythemia is a myeloproliferative neoplasm in which excess platelets are produced, leading to abnormal blood clotting or bleeding. The hands and feet may burn, turn red, and... read more ) 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.


Blood smear

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.

Reticulocyte count

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.

Special tests of 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 Bone Marrow Examination Red blood cells, most white blood cells, and platelets are produced in the bone marrow, the soft fatty tissue inside bone cavities. Sometimes a sample of bone marrow must be examined to determine... read more .

Clotting tests

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.

Measures of proteins and other substances

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 Multiple Myeloma Multiple myeloma is a cancer of plasma cells in which abnormal plasma cells multiply uncontrollably in the bone marrow and occasionally in other parts of the body. People often have bone pain... read more , 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 typing

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