platelet count

Platelet Count 

Definition

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 microliter of blood. Low platelet counts or abnormally shaped platelets are associated with bleeding disorders. High platelet counts sometimes indicate disorders of the bone marrow.

Purpose

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.

Description

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. 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.

Counting methods

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. 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 in contact 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. 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. 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 isn't 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, which will cause the instrument to accidentally miss the platelet. Clumps of platelets will not be counted because clumps exceed the upper size exclusion limit for platelets. 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.

Aftercare

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

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.

Normal results

The normal range for a platelet count is 150,000-450,000 platelets per microliter of blood.

Key terms

Capillaries — The smallest of the blood vessels that bring oxygenated blood to tissues.

EDTA — A colorless compound used to keep blood samples from clotting before tests are run. Its chemical name is ethylene-diamine-tetra-acetic acid.

Hemocytometer — An instrument used to count platelets or other blood cells.

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 — An abnormally low platelet count.

Thrombocytosis — An abnormally high platelet count. It occurs in polycythemia vera and other disorders in which the bone marrow produces too many platelets.

Abnormal results

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 (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. 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.

Resources

Books

Berktow, Robert, et al., editors. Merck Manual of Medical Information. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

---

count (kount) a numerical computation or indication.

---

Addis count  determining the number of erythrocytes, leukocytes, epithelial cells, casts, and protein content in an aliquot of a 12-hour urine specimen.

blood count , blood cell count determining the number of formed elements in a cubic millimeter of blood; it may be a complete blood count or it may measure just one of the formed elements.

complete blood count  a series of tests of the peripheral blood, including the hematocrit, the amount of hemoglobin, and counts of each type of formed element.

differential leukocyte count  a count on a stained blood smear of the proportion of different types of leukocytes (or other cells), expressed in percentages.

platelet count  determination of the total number of platelets per cubic millimeter of blood; the direct platelet c. simply counts the cells using a microscope, and the indirect platelet c. determines the ratio of platelets to erythrocytes on a peripheral blood smear and computes the number of platelets from the erythrocyte count.

---

platelet count,

a blood test that is performed on all patients who develop petechiae, spontaneous bleeding, or increasingly heavy menses. It is also used to monitor the course of the disease or therapy for thrombocytopenia or bone marrow failure.

---

platelet (plāt´let),

n a disc found in the blood of mammals that is involved in the coagulation and clotting of blood.

platelet ac-globulin,

n See factor, platelet, 1.

platelet activating factor,

n 1-0-alkyl-2-sn-glycero-3-phosphocholine. A phospholipid derivative formed by platelets, basophils, neutrophils, monocytes, and macrophages. It is a potent platelet-aggregating agent and inducer of systemic anaphylactic symptoms, including hypotension, thrombocytopenia, neutropenia, and bronchoconstriction.

platelet aggregation,

n a clumping together of platelets in vitro, and likely in vivo, by a number of agents, such as adenosine diphosphate, thrombin, and collagen, as part of a sequential mechanism leading to the initiation and formation of a thrombus or hemostatic plug.

platelet aggregation inhibitors,

n.pl the drugs or agents that antagonize or impair any mechanism leading to blood platelet aggregation.

platelet cofactor I,

n See factor VIII.

platelet cofactor II,

n See factor IX and plasma thromboplastin component.

platelet count,

n the number of platelets found in 1 mm³ of blood; the normal range is between 200,000 and 300,000 platelets.

platelet-derived growth factors (PDGF),

n a type of protein released by platelets of the blood that aid in the repair and regeneration of connective tissue.

platelet disorder,

n See disorder, platelet.

platelet-rich plasma (PRP),

n See plasma, platelet-rich (PRP).

platelet transfusion,

n 1. the transfer of blood platelets from a donor to a recipient or reinfusion to the donor.
n 2. a treatment modality used in treating hemophilia and other conditions of impaired blood coagulation.

---

count

a numerical computation or indication.

---

differential count

a count, on a stained blood smear, of the proportion of different types of leukocytes (or other cells), previously expressed in percentages but now usually reported in absolute numbers (10⁹/l) for a better indication of abnormalities that may exist.

milk cell count

see milk cell counts.

platelet count

the count of the total number of platelets per liter (10⁹/l) of blood by counting the platelets in a counting chamber, a hematology analyzer, or by estimating the number on a stained blood smear.

sperm count

see semen concentration.

total bacterial count

determination of the total number of bacteria in the sample examined microscopically, then a calculation of the number per ml. These do not distinguish between viable and non-viable organisms. See also breed's direct smear method.

viable bacterial cell count

enumerating the number of viable bacteria present in a sample based on counting the number of colonies from a given dilution.

wool count

an arbitrary number given to wool to indicate its fiber diameter, e.g. 60's, based on an eyeball assessment of the number of hanks of yarn that could be spun from one pound of wool. Now superseded by measurement of the diameter, e.g. 20 microns.

worm count

a total worm count requires a freshly slaughtered cadaver, collection of intestinal or other fluid in an aliquot sample; in the case of lungs it is necessary to digest the tissue; counting actual worms and by multiplication measuring the total worm burden.

---

platelet

a small disk or platelike structure, the smallest of the formed elements in blood. Blood platelets (called also thrombocytes) are disk-shaped, non-nucleated blood elements with a very fragile membrane; they tend to adhere to uneven or damaged surfaces. They average about 250,000 per cubic millimeter of blood and are formed in the red bone marrow by fragmentation of megakaryocytes, the largest of the bone marrow cells. Platelet production is controlled by a hormone, thrombopoietin, and regulatory lymphocytes acting at the stem cell level. At any given time about one-third of the total blood platelets can be found in the spleen; the remaining two-thirds are in the circulating blood.

The functions of platelets are related to the clotting of blood. Because of their adhesion and aggregation capabilities platelets can occlude small breaks in blood vessels and prevent the escape of blood. Platelets which have adhered to exposed collagen in damaged vessels release ADP in milliseconds which in turn initiates the synthesis of thromboxane A₂, a very potent prostaglandin which causes platelet aggregation and localized vasoconstriction. Fibrinogen, factors V and VIII, calcium ions, platelet phospholipid (PF-3), associated with the platelet membrane are also released. Substances contained within the platelet granules such as thromboglobulin, heparin neutralizing activity (PF-4) mitogens such as platelet derived growth factor, thrombospondin, ADP, serotonin and calcium ions are also released by aggregated platelets.

---

platelet-activating factor (PAF)

see platelet-activating factor.

platelet adhesion

the adherence of platelets to any area with damaged blood vessels; an important component of hemostasis.

platelet aggregation

the progressive accumulation of platelets, attracted by other platelets once adhesion begins. Thromboxane A₂ causes irreversible platelet aggregation.

platelet aggregation test

a known platelet aggregating factor such as collagen, ADP or thrombin is added to a suspension of the platelets under test and the degree of aggregation measured by decrease in turbidity of the suspension.

platelet count

may be performed directly (in a hemocytometer chamber) or indirectly (estimating from the stained blood smear by number per field or in comparison to the number of white blood cells), expressed as number of cells per liter of blood.

platelet-derived growth factor

one of three growth factors released by platelets which undergo the release reaction; the growth factors stimulate endothelial cell proliferation.

platelet distribution width (PDW)

an indication of variation in platelet size which can be a sign of active platelet release.

platelet factor 3 (PF-3) test, platelet release test

test the antiplatelet activity of serum; used to detect circulating antiplatelet antibodies. Antibody-antigen reactions involving platelets cause the release of PF-3 from platelets which in turn shortens the contact-activated clotting time of platelet-rich plasma (PRP).

platelet factors

factors important in hemostasis which are contained in or attached to the platelets: platelet factor 1 is adsorbed clotting factor V from the plasma; platelet factor 2 is an accelerator of the thrombin-fibrinogen reaction; platelet factor 3 is a phospholipid with potent procoagulant activity; platelet factor 4 is capable of inhibiting the activity of heparin (heparin neutralizing activity).

mean platelet volume (MPV)

elevated level is an indication of increased megakaryocyte shedding of platelets and decreased level is seen in thrombocytopenia.

platelet plug formation

see platelet aggregation (above).

platelet release reaction

measured by the degree of secondary ADP-mediated aggregation that occurs. This is assessed by the amount of PF-4, PF-3 or serotonin, etc. released.

platelet retention

tested by testing the adhesiveness of a suspension of the subject platelets to a glass bead column or standard size filter.

platelet rich plasma

plasma prepared by centrifugation to separate out red blood cells but not platelets for transfusion.

platelet storage-pool disease

an inherited autosomal thrombopathia in American foxhounds and cats characterized by a deficiency of platelet storage granules.

platelet transfusion

transfusion of fresh, nonchilled whole blood is the usual method of transfusing platelets to an animal with thrombocytopenia.

---

platelet count

Enumeration of circulating platelets Specimen Blood, EDTA–lavender top-tube Ref range 150-450 x 109/L–US: 150-400 000/µL–mm3; 'panic' values: < 20 x 109/L–US: < 20 000/µL; > 1000 x 109/L–US: 1,000,000/µL ↑ in Myeloproliferative disorders and/or polycythemia vera, and less commonly with infections, blood loss, and splenectomy; rare causes of ↑ PC include anemia–hemolytic, iron-deficiency,
sickle cell, cirrhosis, collagen vascular disease, cryoglobulinemia, drugs–
epinephrine, OCs, exercise, hemorrhage, hypoxia, ITP, post-partum, pregnancy, rheumatoid arthritis, TB ↓ in < 0.02 x 1012/L–US: 20 000/mm3–µL are associated with ↑ bleeding tendency; platelets are ↓ in malignacies of bone, GI tract, brain, leukemia, kidney or liver disease, aplastic anemia, DIC, ITP, SLE, drugs associated with ↓ PC: aspirin, chemotherapeutic agents, chloromycetin, phenylbutazone, quinidine, thiazide diuretics, tolbutamide. See Platelet concentrate.