A platelet count is a diagnostic test that determines the number of plateletsin the patient's blood. Platelets, which are also called thrombocytes, are small disk-shaped blood cells produced in the bone marrow and involved in theprocess of blood clotting. There are normally between 150,000-450,000 platelets in each microliter of blood. Low platelet counts or abnormally shaped platelets are associated with bleeding disorders. High platelet counts sometimesindicate 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 diseases such as leukemia, polycythemia vera, or aplastic anemia are given periodic platelet count tests to monitor their health.
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 5mL of blood are drawn from a vein in the patient's inner elbow region. Blooddrawn 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 afterremoval from the body. The changing volume is relatively stable for a periodof one to three hours after collection. This period is the best time to countthe sample when using electronic instruments, because the platelets will bewithin 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 microscopeslide and spreading it into a thin layer. After staining to make the variousblood cells easier to see and distinguish, a laboratory technician views thesmear through a light microscope. Accurate assessment of the number of platelets requires other methods of counting. There are three methods used to countplatelets; hemacytometer, voltage-pulse counting, and electro-optical 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 guidethe counting. For a proper count, the platelets should be evenly distributedin the hemacytometer. Counts made from samples with platelet clumping are considered unreliable. Clumping can be caused by several factors, such as clotting before addition of the anticoagulant and allowing the blood to remain incontact with a capillary blood vessel during collection. Errors in platelet counting are more common when blood is collected from capillaries than from veins.
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 theblood through an electronic counter. The instruments are set to count only particles within the proper size range for platelets. The upper and lower levels of the size range are called size exclusion limits. Any cells or material larger or smaller than the size exclusion limits will not be counted. Any object in the proper size range is counted, however, even if it is not a platelet. 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 because of the size exclusion limits, whichwill cause the instrument to accidentally miss the platelet. Clumps of platelets will not be counted because clumps exceed the upper size exclusion limitfor platelets. In addition, if the patient has a high white blood cell count, electronic counting may yield an unusually low platelet count because whiteblood 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, theirfragments will be falsely counted as platelets.
Risks for a platelet count test are minimal in normal individuals. Patients 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.
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. Hypersplenism is characterized by the collection 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. All of these diseases produce reduced platelet counts.
Abnormally high platelet levels (thrombocytosis) may indicate either a benignreaction to an infection, surgery, or certain medications; or a disease likepolycythemia vera, in which the bone marrow produces too many platelets tooquickly.