COVID-19 alters platelet activation and increases PLAs. (A) Platelet P-selectin expression was measured in whole blood by flow cytometry at baseline in 17 healthy donors and 5 non-ICU and 12 ICU COVID-19 patients. Blue dots indicate ICU patients; red dots indicate non-ICU patients. (B) P-selectin expression was measured after activation by platelet agonists. P2Y12 activation was induced by 1 ng/mL 2MeSADP, PAR1 was activated through 2.5 μM SFLLRN (TRAP), and GPVI was activated with 1 ng/mL CRP (N = 17-18 per group). Blue dots indicate ICU patients; red dots indicate non-ICU patients. (C-F) PLAs were measured in whole blood by flow cytometry. N = 7 for the healthy donors and N = 12-13 for COVID-19 patients. Blue dots indicate ICU patients; red dots indicate non-ICU patients. (C) PNAs were identified as CD66b⁺CD41⁺ leukocytes. (D) Platelet-monocyte aggregates were identified as CD14⁺CD41⁺ leukocytes. (E) Platelet-CD4 T-cell aggregates were identified as CD3⁺CD4⁺CD41⁺ leukocytes. (F) Platelet-CD8 T-cell aggregates were identified as CD3⁺CD8⁺CD41⁺ leukocytes. *P < .05; ***P < .001