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Laboratory Tests for Blood Disorders


David J. Kuter

, MD, DPhil, Harvard Medical School

Last full review/revision Jun 2021| Content last modified Jun 2021
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Doctors select tests to help diagnose blood disorders based on the person's symptoms and the results of the physical examination. 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). 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, 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 folate (the vitamin folic acid) or a deficiency 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 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).

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.


Complete Blood Count (CBC)*


What It Measures

Normal Values


Amount of this oxygen-carrying protein within a volume of blood

Men: 14 to 17 grams per deciliter (140 to 170 grams/L)

Women: 12 to 16 grams per deciliter (120 to 160 grams/L)


Proportion of the total amount of blood (blood volume) made up of red blood cells (plasma makes up the rest)

Men: 41 to 51%

Women: 36 to 47%

Mean cellular (or corpuscular) volume (MCV)

Average volume of a red blood cell

80 to 100 femtoliters per cell

Mean cellular (or corpuscular) hemoglobin (MCH)

Amount of hemoglobin per red blood cell

28 to 32 picograms per cell

Mean cellular (or corpuscular) hemoglobin concentration (MCHC)

Average concentration of hemoglobin within red blood cells

32 to 36 grams per deciliter of red blood cells (320 to 360 grams per liter)

Red blood cell (RBC) count

Number of red blood cells in a volume of blood

Men: 4.5 to 5.9 million cells per microliter (4.5 to 5.9 × 1012/L)

Women: 4.0 to 5.2 million cells per microliter (4.05 to 5.2 × 1012/L)

Red cell distribution width

Amount of variability in the sizes of the red blood cells

11.5 to 14.5%

White blood cell count

Number of white blood cells in a specified volume of blood

4,500 to 11,000 per microliter (4.5 to 11 × 109/L)

Differential white blood cell count

Percentages and numbers of the different types of white blood cells

Segmented neutrophils: 40 to 70%, or 1800 to 7700 per microliter (1.8 to 7.7 x 109/L)

Lymphocytes: 22 to 44%, or 1000 to 4800 per microliter (1 to 4.8 × 109/L)

Monocytes: 4 to 11%, or 200 to 1200 per microliter (0.2 to 1.2 × 109/L)

Eosinophils: 0 to 8%, or 0 to 900 per microliter (0 to 0.9 × 109/L)

Basophils: 0 to 3%, or 0 to 300 per microliter (0 to 0.3 × 109/L)

Platelet count

Number of platelets in a specified volume of blood

140,000 to 450,000 per microliter (140 to 450 × 109/L)

* Normal values vary from laboratory to laboratory.

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 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.

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, 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 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, B12 and folate) that are necessary for the production of healthy blood cells also can be measured.

Blood typing

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.

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