A platelet count is a diagnostic test that determines the number of platelets in the patient's blood. Platelets, which are also called thrombocytes, are small disk-shaped blood cells produced in the bone marrow and involved in the process of blood clotting. There are normally between 150,000–450,000 platelets in each micro-liter of blood. Low platelet counts or abnormally shaped platelets are associated with bleeding disorders. High platelet counts or low platelet counts sometimes indicate disorders of the bone marrow.
The primary functions of a platelet count are to assist in the diagnosis of bleeding disorders and to monitor patients who are being treated for any disease involving bone marrow failure. Patients who have leukemia, polycythemia vera, or aplastic anemia are given periodic platelet count tests to monitor their health.
Blood collection and storage
Platelet counts use a freshly collected blood specimen to which a chemical called EDTA has been added to prevent clotting before the test begins. About 5 mL of blood are drawn from a vein in the patient's inner elbow region, or other area. Blood drawn from a vein helps to produce a more accurate count than blood drawn from a fingertip. Collection of the sample takes only a few minutes. After collection, the mean platelet volume of EDTA-blood will increase over time. This increase is caused by a change in the shape of the platelets after removal from the body. The changing volume is relatively stable for a period of one to three hours after collection. This period is the best time to count the sample when using electronic instruments, because the platelets will be within a standard size range.
Platelets can be observed in a direct blood smear for approximate quantity and shape. A direct smear is made by placing a drop of blood onto a microscope slide and spreading it into a thin layer. After staining to make the various blood cells easier to see and distinguish, a laboratory technician views the smear through a light microscope. Accurate assessment of the number of platelets requires other methods of counting. There are three methods used to count platelets: hemacytometer, voltage-pulse counting, and electro-optical counting.
Hemacytometer counting: The microscopic method uses a phase contrast microscope to view blood on a hemacytometer slide. A sample of the diluted blood mixture is placed in a hemacytometer, which is an instrument with a grid etched into its surface to guide the counting. For a proper count, the platelets should be evenly distributed in the hemacytometer. Errors in platelet counting are more common when blood is collected from capillaries than from veins.
Electronic counting: Electronic counting of platelets is the most common method. There are two types of electronic counting, voltage-pulse and electro-optical counting systems. In both systems, the collected blood is diluted and counted by passing the blood through an electronic counter. For these instruments to work properly, the sample must not contain other material that might mistakenly be counted as platelets. Electronic counting instruments sometimes produce artificially low platelet counts. If a platelet and another blood cell pass through the counter at the same time, the instrument will not count the larger cell, which will cause the instrument to accidentally miss the platelet. Clumps of platelets will also not be counted. In addition, if the patient has a high white blood cell count, electronic counting may yield an unusually low platelet count because white blood cells may filter out some of the platelets before the sample is counted. On the other hand, if the red blood cells in the sample have burst, their fragments will be falsely counted as platelets.
Because platelet counts are sometimes ordered to diagnose or monitor bleeding disorders, patients with these disorders should be cautioned to watch the puncture site for signs of additional bleeding.
Risks for a platelet count test are minimal in normal individuals. Children with bleeding disorders, however, may have prolonged bleeding from the puncture wound or the formation of a bruise (hematoma) under the skin where the blood was withdrawn. Rarely an infection may occur at the needle puncture site.
Capillaries —The tiniest blood vessels with the smallest diameter. These vessels receive blood from the arterioles and deliver blood to the venules. In the lungs, capillaries are located next to the alveoli so that they can pick up oxygen from inhaled air.
EDTA —A colorless compound used to keep blood samples from clotting before tests are run.
Hemocytometer —An instrument used to count platelets or other blood cells.
Leukemia —A cancer of the blood-forming organs (bone marrow and lymph system) characterized by an abnormal increase in the number of white blood cells in the tissues. There are many types of leukemias and they are classified according to the type of white blood cell involved.
Phase contrast microscope —A light microscope in which light is focused on the sample at an angle to produce a clearer image.
Thrombocyte —Another name for platelet.
Thrombocytopenia —A persistent decrease in the number of blood platelets usually associated with hemorrhaging.
Thrombocytosis —An abnormally high platelet count. It occurs in polycythemia vera and other disorders in which the bone marrow produces too many platelets.
The normal range for a platelet count is 150,000–450,000 platelets per microliter of blood.
An abnormally low platelet level (thrombocytopenia) is a condition that may result from increased destruction of platelets, decreased production, or increased usage of platelets. In idiopathic thrombocytopenic purpura (ITP), platelets are destroyed at abnormally high rates. Another cause of a low platelet count is an enlarged spleen. Hypersplenism is characterized by the collection (sequestration) of platelets in the spleen. Disseminated intravascular coagulation (DIC) is a condition in which blood clots occur within blood vessels in a number of tissues. Leukemia and aplastic anemia can result in a low platelet count because of decreased production of platelets in the bone marrow. All of these diseases produce reduced platelet counts. Abnormally high platelet levels (thrombocytosis) may indicate either a benign reaction to an infection, surgery, or certain medications; or a disease like polycythemia vera, in which the bone marrow produces too many platelets too quickly.
The pain from the needle puncture only lasts a moment. The parent should comfort a child as needed. Older children can be prepared for the test ahead of time, and the reason why the test is being given should also be explained if the child is old enough to understand.
When to call a doctor
If the bleeding does not stop at the needle puncture site, or hours to days later, there are signs of infection (redness and swelling), then parents should contact a doctor.
Henry, John B. Clinical Diagnosis and Management by Laboratory Methods, 20th Ed. Philadelphia: W. B. Saunders Co., 2001.
Wallach, Jacques. Interpretation of Diagnostic Tests, 7th Ed. Philadelphia: Lippincott, Williams, and Wilkins, 2000.
Mark A. Best