coat platelet defect coat platelet defect a collagen- and thrombin-activated (coat) platelet defect is a platelet function disorder that is due to a reduced ability to generate procoagulant platelets. it is associated with a clinically relevant bleeding phenotype. during physiological platelet activation, a fraction of platelets expresses phosphatidylserine on their surface and become highly efficient in sustaining thrombin generation. these so-called coat platelets, can be generated by dual-agonist stimulation with collagen and thrombin in a laboratory setting. coat platelet defects should be distinguished from scott syndrome, a rare bleeding disorder in which patients have impaired phospholipid scrambling and do not express negatively charged phospholipids on their surface even after treatment with calcium ionophores.