PF4-heparin ELISA OD and platelet count in patients with COVID-19. (A) Results of PF4-heparin ELISA OD are given in relation to the WHO Severity Score of COVID-19 disease in 222 patients with COVID-19. ELISA cutoff, 0.5 OD. The 10 patients who developed thrombosis are indicated by open circles. Solid symbols indicate the 212 patients without thrombosis. There was no correlation between WHO severity score of COVID-19 disease and reactivity on the PF4-heparin ELISA. (B) Platelet counts in COVID-19 patients with and without thrombosis.