Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

Assessment of platelet procoagulant function by flow cytometry is increasingly recognized for diagnosing platelet disorders. Induction and accurate detection of phosphatidylserine exposure on the platelet surface requires proper recalcification of citrated blood samples, particularly in plasma from thrombocytopenic patients, in which residual citrate can limit free extracellular calcium availability. To define optimal recalcification conditions for reliable measurement of procoagulant Collagen-And-Thrombin–activated (COAT) platelets in platelet-rich plasma (PRP) samples with low platelet counts. Fresh PRP from healthy donors was diluted with autologous platelet-poor plasma to simulate platelet counts from 160 × 10⁹/L to 15 × 10⁹/L. Undiluted PRP (control) and diluted samples were spiked with varying calcium concentrations and stimulated with convulxin and thrombin. Flow cytometry was used to measure annexin V and PAC-1 binding to assess procoagulant platelet generation. For PRP with platelet counts ≥100 × 10⁹/L, no addition of calcium beyond the 2.5 mM already present in the buffer was required. For platelet counts between 30 and <100 × 10⁹/L, supplementation with 3 mM calcium restored procoagulant platelet generation to 85% to 115% of undiluted PRP control levels. Counts of 20 to 5 mM) impaired platelet function, highlighting the need to avoid excessive recalcification. Optimized PRP recalcification prevents underestimation of procoagulant platelet potential in thrombocytopenic samples. This practical approach addresses a key gap identified by the International Society on Thrombosis and Haemostasis Scientific and Standardization Committee and helps laboratories to ensure reliable platelet function testing in PRP samples from thrombocytopenic patients.