Introduction Defects of platelet functional responses in COVID-19 were reported, but their origin and pathophysiological significance are unclear. The objective of this study was to characterize the thrombocytopathy in COVID-19. Materials and methods Analysis of platelet functional responses to activation by flow cytometry and aggregometry in 46 patients with confirmed COVID-19 of different severity (non-ICU, ICU, and ECMO) over the course of hospitalization alongside with plasma coagulation, inflammatory markers (CRP, fibrinogen, NETosis assays in smears) was performed. Results and conclusions All patients had increased baseline percentage of procoagulant platelets (healthy: 0.9 ± 0.5%; COVID-19: 1.7 ± 0.6%). Patients had decreased agonist-induced platelet GPIb shedding (1.8 ± 0.7 vs 1.25 ± 0.4), P-Selectin exposure (1.51 ± 0.21 vs 1.1 ± 0.3) and aggregation. The values of these parameters among the non-ICU and ICU cohorts differed modestly, while the ECMO cohort differed significantly. Only ECMO patients had pronounced thrombocytopenia. While inflammatory markers improved over time, the observed platelet functional responses changed only moderately. SARS-CoV-2 RNA was found in 8% of blood samples and it did not correlate with platelet counts or responses. All patients had increased NETosis that moderately correlated with platelet dysfunction. High cumulative dosages of LMWH (average > 12,000 IU/day over 5 days) resulted in an improvement in platelet parameters. The observed pattern of platelet refractoriness was reproduced by in vitro pre-treatment of washed platelets with subnanomolar thrombin or perfusion of blood through a collagen-covered flow chamber. We conclude that platelet dysfunction in COVID-19 is consistent with the intravascular-coagulation-induced refractoriness rather than with an inflammation-induced mechanism or a direct activation by the virus.

It is known that in COVID-19, hypercoagulation and sometimes thrombocytopenia are related to disease severity. There is also controversial data on platelet participation in COVID-19 pathology. We aimed to determine the degree of platelet hyperactivation in COVID-19 patients. Whole blood flow cytometry with Annexin-V and lactadherin staining ("PS+ platelets") was utilized. Additionally, a stochastic mathematical model of platelet production and consumption was developed. Here we demonstrated that the percentage of PS+ platelets in COVID-19 patients was twofold that of healthy donors. There was a significant correlation between the amount of PS+ platelets and the percentage of lung damage in patients. No connection was found between platelet senescence and hospital therapy or patients' chronic diseases, except for chronic lung disease. Although no thrombocytopenia was observed in patients, the observed increase in platelet size (FSC-A parameter in flow cytometry) could indicate that platelet age is decreased in patients. The developed computational model of platelet turnover confirms the possibility of intense platelet consumption without noticeable changes in platelet count. We conclude that the observed platelet hyperactivation in COVID-19 could be caused by platelet activation in circulation, leading to platelet consumption without significant thrombocytopenia.