Platelet count is the number of platelets (thrombocytes) found in a cubic milliliter of human bloodblood. Platelet Count is ordered routinely as a prat of Complete Blood Cell Count. Some types of anemia affect the Platelet Count as they do with Red Blood Cell Count, Hemoglobin Level, Hematocrit Levels, and RBC Indecies. Platelet Count is very helpful when it comes to monitoring the progress of therapy for Bone Marrow Failure and case of Thrombocytopenia. Platelet count is usually performed on patients who develop Petechiae which are small hemorrhages occur on the skin. The test is also performed on patients with Spontaneous Bleeding, and Thrombocytopenia (Low Platelet Count).
Like other blood cells, Platelets are formed in the bone marrow. In bone marrow Platelets are differentiated from other cells known as Megakaryocytes. Platelets are small, round, nonnucleated cells whose main function is to maintain vascular integrity. In blood vessel injury, hemostasis is required to form a clot that will durably plug the hole until healing can occur. The primary phase of the hemostatic mechanism involves platelet aggregation. From there, the platelets help initiate the coagulation factor cascade. Majority of the platelets are found in the bloodstream. And a smaller percentage (25%) exists in the liver and spleen. Survival of platelets is measured in days. The average survival rate of platelets is 7 to 9 days.
Platelet activity is essential to blood clotting. Counts of 150,000 to 400,000/mm3 are considered normal. Counts of less than 100,000/mm3 are considered to indicate thrombocytopenia; thrombocytosis is said to exist when counts are greater than 400,000/mm3Thrombocythemia is a term used to indicate a platelet count in excess of 1 million/mm3. Vascular thrombosis with tissue or organ infarction is the major complication of thrombocythemia. The most common diseases associated with spontaneous thrombocytosis are iron deficiency anemia and malignancy (leukemia, lymphoma, solid tumors such as of the colon). Thrombocytosis may also occur with polycythemia vera and postsplenectomy syndromes. It should be noted that even patients with elevated platelet counts can experience a bleeding tendency because the function (platelet aggregation) of those platelets may be abnormal. It is not uncommon for patients whose platelet counts exceed 1 million to experience spontaneous bleeding and thrombocytosis.
Spontaneous hemorrhage may occur with thrombocytopenia. If thrombocytopenia is severe, the platelets are often hand counted. Spontaneous bleeding is a serious danger when platelet counts fall below 20,000/mm3. Petechiae and ecchymosis will also occur at that degree of thrombocytopenia. With counts above 40,000/mm3, spontaneous bleeding rarely occurs, but prolonged bleeding from trauma or surgery may occur with counts at this level.
Causes of thrombocytopenia include:
- Reduced production of platelets (secondary to bone marrow failure or infiltration of fibrosis, tumor, etc.)
- Sequestration of platelets (secondary to hypersplenism)
- Accelerated destruction of platelets (secondary to antibodies, infections, drugs, prosthetic heart valves)
- Consumption of platelets (secondary to disseminated intravascular coagulation [DIC])
- Platelet loss from hemorrhage
- Dilution with large volumes of blood transfusions containing very few, if any, platelets
A List of Causes of False Platelet Count
- Living in high altitudes may cause increased platelet levels. This is also true to Red Blood Cell Count, Hemoglobin Levels, and Hematocrit Levels
- Because platelets can clump together, automated counting is subject to at least a 10% to 15% error.
- Strenuous exercise may cause increased levels.
- Decreased levels may be seen before menstruation.
- Drugs that may cause High Platelet Counts include Estrogens and Oral Contraceptives.
- Drugs that may cause Low Platelet Counts include Chemotherapeutic Agents, Chloramphenicol, Colchicine, Histamine-2 blocking Agents ( Including Cimetidine and Zantac), Hydralazine, Indomethacin, Isoniazid (INH), Quinidine, Streptomycin, Sulfonamides, Thiazide Diuretics, and Tolbutamide (Orinase).
Normal Platelet Count
Newborn: 150,000-300,000/mm3 (μL)
Premature infant: 100,000-300,000/mm3
Infant: 200,000-475,000 mm3
Causes of High Platelet Count
High Platelet Count is known as Thrombocytosis. Thrombocytosis is usually accompanied with abnormal increases or decreases in other blood componentes (Red Blood Cells, and White Blood Cells) which makes Platelet Count as important as the other members of Complete Blood Count when it comes to understanding the underlying causes behind abnormalities with other blood cells.
- High Platelet Count is associated with Malignant Disorders including Leukemia, Lymphoma, and Solid Tumors (such as of the colon). The pathophysiology behind this relationship is not known however.
- Polycythemia Vera: This is a hyperplasia of all the marrow cell lines, including platelets.
- Postsplenectomy Syndrome: This occurs after performing Splenectomy which is a surgical procedure in which the Spleen is partially or fully removed. The Spleen works with other organs (the Liver for example) as filters that remove old blood cells from the blood stream. A partial or full removal of Spleen will cause some of these aging blood cells to skip filtering which leads to increased count of blood cells including the Platelet Count.
- Rheumatoid Arthritis: The pathophysiology behing the relationship between Rheumatoid Arthritis and High Platelet Count is not known.
- Iron-deficiency Anemia or Following Hemorrhagic Anemia: Even though Iron doesn’t play any role in the production of platelets, Anemia causes maximal stimulation of cellular production by the marrow. As the marrow increase the production of blood cells. Red blood cells production will not be increased since there is not sufficient iron available to produce more Red Blood Cells. The platelet, however, can easily respond even in the presence of iron deficiency.
Causes of High Platelet Count
- Hypersplenism: This is the condition of enlarged spleen. Since the spleen normally filters and removes old platelets from the bloodstream, when it is abnormally enlarged, more platelets, including new ones, will be caught in the process and removed from the blood stream which reduces the Platelet count over time.
- Hemorrhage cause bleeding which lease to platelets loss from the the bleeding. If not replaced by transfusion of platelets, it will take some time (hours to days) for the marrow to produce an adequate number of platelets. This problem is exacerbated with treatment that replenishes blood volume and RBC count. This treatment dilutes the remaining platelets and further decreases the platelet count.
- Immune thrombocytopenia (e.g., idiopathic thrombocytopenia, neonatal, posttransfusion, or drug-induced thrombocytopenia): Antibodies directed against antigens on the platelet cell membrane destroy the platelet and the count decreases.
- Leukemia and other myelofibrosis disorders: The marrow is replaced by neoplastic or fibrotic tissue. Megakaryocyte function and numbers diminish. Platelets are not produced, and the count drops.
- Thrombotic thrombocytopenia: This disease and others such as HELLP (hemolysis [H], elevated liver enzymes [EL], low platelet count [LP]) syndrome are highlighted by thrombocytopenia, hemolytic anemia, and other hematologic abnormalities.
- Graves Disease: Sometimes, thrombocytopenia occurs to small number of patients who have Graves Disease. However, the pathophysiology behind this observation is not known.
- Inherited Disorders including Wiskott-Aldrich, Bernard-Soulier, and Zieve Syndromes: The pathophysiology of this observation is also not known.
- DIC: The pathophysiology of thrombocytopenia is not clear. In part, however, it is thought that ongoing thrombosis “consumes” the platelets much like coagulating factors are “consumed.” DIC usually develops concurrently with other severe disease (e.g., gram-negative sepsis) that can also produce thrombocytopenia.
- Systemic lupus erythematosus: The pathophysiology of this observation is not known.
- Pernicious anemia: Unlike iron, vitamin B12 is required when it comes to producing Platelets. A deficiency of this vitamin or folate will diminish the production of platelets.
- Hemolytic anemia: Lysis of Red Blood Cells (Hemolysis) is often accompanied with lysis of Platelets. This is because that conditions that lead to hemolysis lead to destruction of Platelets which decrease the Platelet Count.l
- Cancer chemotherapy: Cytotoxic drugs often affect the bone marrow. Platelets are not produced at adequate levels, and the count drops.
- Infection: Bacterial, viral, and rickettsial infections can cause thrombocytopenia. This ofter occur in high rates in patients who are immunocompromised as in the case of Acquired Immunodeficiency Syndrome (AIDS) for instance.