Heparin resistance can be defined as the requirement of more than 35,000 units of UFH in 24 hours to reach therapeutic aPTT levels and is classically caused by antithrombin III (ATIII) deficiency.18ATIII-independent forms of heparin resistance (also called “apparent,” or “pseudo” heparin resistance) occur as well, possibly caused by high concentrations of FVIII, fibrinogen, or platelets.19, 20, 21Apparent heparin resistance is assumed to be an in vitro effect only, but the mechanisms and clinical meaning of this phenomenon are not fully discerned and may not apply to COVID-19 patients. Here, SERPINC1 is linked to COVID-19.