Platelet-activating immune complexes identified in critically ill COVID-19 patients suspected of heparin-induced thrombocytopenia

J Thromb Haemost. 2021 May;19(5):1342-1347. doi: 10.1111/jth.15283. Epub 2021 Mar 14.


Background: Thrombocytopenia and thrombosis are prominent in coronavirus disease 2019 (COVID-19), particularly among critically ill patients; however, the mechanism is unclear. Such critically ill COVID-19 patients may be suspected of heparin-induced thrombocytopenia (HIT), given similar clinical features.

Objectives: We investigated the presence of platelet-activating anti-platelet-factor 4 (PF4)/heparin antibodies in critically ill COVID-19 patients suspected of HIT.

Patients/methods: We tested 10 critically ill COVID-19 patients suspected of HIT for anti-PF4/heparin antibodies and functional platelet activation in the serotonin release assay (SRA). Anti-human CD32 antibody (IV.3) was added to the SRA to confirm FcγRIIA involvement. Additionally, SARS-CoV-2 antibodies were measured using an in-house ELISA. Finally, von Willebrand factor (VWF) antigen and activity were measured along with A Disintegrin And Metalloprotease with ThromboSpondin-13 Domain (ADAMTS13) activity and the presence of anti-ADAMTS13 antibodies.

Results: Heparin-induced thrombocytopenia was excluded in all samples based on anti-PF4/heparin antibody and SRA results. Notably, six COVID-19 patients demonstrated platelet activation by the SRA that was inhibited by FcγRIIA receptor blockade, confirming an immune complex (IC)-mediated reaction. Platelet activation was independent of heparin but inhibited by both therapeutic and high dose heparin. All six samples were positive for antibodies targeting the receptor binding domain (RBD) or the spike protein of the SARS-CoV-2 virus. These samples also featured significantly increased VWF antigen and activity, which was not statistically different from the four COVID-19 samples without platelet activation. ADAMTS13 activity was not severely reduced, and ADAMTS13 inhibitors were not present, thus ruling out a primary thrombotic microangiopathy.

Conclusions: Our study identifies platelet-activating ICs as a novel mechanism that contributes to critically ill COVID-19.

Keywords: COVID-19; antigen-antibody complex; heparin-induced thrombocytopenia; thrombocytopenia; thrombosis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anticoagulants
  • Antigen-Antibody Complex
  • COVID-19*
  • Critical Illness
  • Heparin / adverse effects
  • Humans
  • Platelet Factor 4
  • SARS-CoV-2
  • Thrombocytopenia* / chemically induced
  • Thrombocytopenia* / diagnosis


  • Anticoagulants
  • Antigen-Antibody Complex
  • Platelet Factor 4
  • Heparin