Clot retraction kinetics using platelets from an afibrinogenemic patient. Platelets (1.6 × 10⁸mL⁻¹ final concentration) were preincubated for 10 minutes at 37°C with 300 nmol/L final concentration of the indicated fibrinogen before adding human α-thrombin to 0.5 U/mL final concentration. The retraction was filmed on videotape, which was used to generate computer images of the clots at 2-minute intervals. The length and width of the computer images were measured with a ruler and used to calculate clot area, which was then used to calculate % retraction. Percent retraction is plotted versus time for the various recombinant fibrinogens, which are indicated in the legend (N = 1). (□), Normal recombinant; (▪), Aα DE²/γ407 fibrinogen.