Platelet Count



Platelet Count



Definition


Platelets (thrombocytes) are the smallest of all the formed elements in the blood. Counting may be performed directly (in a hemocytometer chamber) or indirectly (estimating from the stained blood smear by number per field or in comparison with the number of white blood cells), or by a quantitative buffy coat (QBC) method, expressed as the number of cells per liter of blood. Automated methods generally are not accurate in exotic species, especially for avian and reptile thrombocytes. The Natt-Herrick method is most common for manual counting. Blood is diluted 1 : 200 and is applied to a charged Neubauer-ruled hemocytometer. The number of platelets/thrombocytes is counted in the large central grid on both sides, and this value is multiplied by 1000 to obtain the number per µL of blood.




Physiology


Platelets are cytoplasmic fragments formed from megakaryocytes in the bone marrow. They are the smallest of the cellular components of the blood in mammals and contain numerous cytosolic organelles and granules, including lysosomes, alpha granules (thromboglobulin, von Willebrand factor, fibrinogen), and dense bodies (adenine, serotonin, calcium). Avian and reptile patients lack platelets but contain thrombocytes—small nucleated cells believed to be derived from the monocyte lineage. The main function of both platelets and thrombocytes involves primary hemostasis. This process is best understood in platelets, whereby vascular damage exposes subendothelial basement membrane collagen, creating a surface for platelet adhesion via surface receptors (glycoprotein 1b) in association with von Willebrand factor. Binding causes activation of platelets with release of cytoplasmic granules and conformational changes to allow additional platelet-platelet bridges to form via fibrinogen and additional surface receptors (glycoprotein IIb/IIIa). This primary hemostatic plug is then stabilized from activation of coagulation with the ultimate formation of cross-linked fibrin. Thrombocytes are believed to function in a similar manner and are also capable of phagocytosis. Because platelets are consumed during the clotting process, conditions accompanied by excessive clotting (sepsis with disseminated intravascular coagulation) are associated with acute reductions in platelet numbers (thrombocytopenia). Platelets and thrombocytes can be destroyed by immune-mediated mechanisms that target their surface receptors. Absolute increases in platelet numbers (thrombocytosis) normally reflect excessive production through nonspecific bone marrow stimulation, sometimes resulting from malignancy.




Clinical Applications





Causes of Abnormally low Levels


Decreased platelet/thrombocyte numbers (thrombocytopenia) result from impaired production or increased rate of destruction. Isolated defects in platelet production are rare and usually are drug induced (alcohols, carbamazepine). More commonly, platelet production is reduced as part of trilineage marrow suppression. This can result from aplastic anemia, myelodysplasias or hematopoietic malignancies, granulomatous diseases, or marrow suppression from chemotherapy or radiation. Accelerated loss is commonly due to consumption from disseminated intravascular coagulation (DIC) or severe blood loss/anemia, but it is also seen with immune-mediated destruction (autoimmune [ITP], alloimmune, drug-induced) or microangiopathic destruction (thrombotic thrombocytopenic purpura [TTP]).

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Jul 28, 2016 | Posted by in EXOTIC, WILD, ZOO | Comments Off on Platelet Count

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