An example of such an effect is the binding of FVIII and acute phase proteins to UFH, resulting in a discordancy between aPTT and anti-Xa.35Increased plasma concentrations of fibrinogen or FVIII may lead to aPTT shortening in 16% of patients with COVID-19.36We believe that in such cases, the aPTT may be a more representative test to assess the net effect of heparin on the in vivo coagulation and may therefore be superior to the anti-Xa. This evidence concerns the gene TNXA and COVID-19.