UHS and UTHSCSA
Department of Pathology
Platelets and the PFA-100
By Andrea Yunes
Platelets are anucleated cytoplasmic fragments of megakaryocytes. Three zones are present: peripheral zone, sol-gel zone, and organelle zone. The peripheral zone functions as a stimulus receptor/transmitter zone and imparts the HLA compatibility and ABO specificity. The sol-gel zone is the contractile portion of the platelet made of microtubules and microfilaments. The organelle zone contains granules (alpha, dense, etc), mitochondria and the dense tubular system.
Platelets are responsible for primary hemostasis by adhesion, activation, secretion, and aggregation. They also help maintain vascular integrity by releasing substances such as platelet-derived growth factor at the site of vascular injury. Fluid dynamics play an important part in platelet function. The velocity of blood near the wall of a vessel is lower than towards the center. The difference in velocity creates shear stress between parallel layers of fluid. This shear stress is greatest at the vessel wall, where platelets and other cells in the blood tend to circulate. A number of changes occur with increasing shear stress. One such change is the GP1b receptor on platelets increases its affinity for von Willebrand factor on endothelial cells. This allows fast association so the platelet is tethered to the vessel wall. Quickly after it is tethered, the platelet dissociates and “rolls” along the vessel wall while it becomes activated and can adhere irreversibly to the vessel wall.
The Platelet Function Analzyer-100 is a fairly new tool used to test platelet function at high shear stress to simulate primary hemostasis. Platelet adhesion and aggregation are initiated by exposure to either collagen/ADP or collagen/epinephrine at high shear stress through a capillary and aperture. The time until plug formation occurs at the aperture is measured in seconds (closure time CT). If CT for both cartridges is prolonged, then a platelet function abnormality is suspected. If CT for only epinephrine is prolonged, aspirin effect is likely since aspirin inhibits platelet aggregation and section in response to epinephrine. The test is not sensitive to oral anticoagulation, heparin, fibrinogen, or clotting factor deficiencies. However, it is sensitive to hematocrit and platelet count. There are many advanatages to the PFA-100 including: ease of operation, no sample preparation, minimal operator interaction, rapid results, and self-contained, single unit test cartridges. Many feel it is superior to bleeding time to test for a primary hemostasis disorder.
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Last updated on 02 May 2007 by John D. Olson, M.D.,