Fibronectin inhibits platelet activation in the HIPA test. Washed platelets were incubated with reviparin (0.2 aFXaU/mL) and anti-PF4/heparin positive sera in the absence or presence of increasing concentrations of fibronectin. Lag-time of platelet activation was monitored for 45 minutes. No observation of platelet activation within 45 minutes was recorded as 45 minutes. Data are mean ± SD of 4 sera tested with 4 platelet donors each (n = 16 donors in total).