Elevated fibrinogen increases clot stability. (A) Human PRP and PPP prepared from CTI-inhibited whole blood were spiked with fibrinogen (to 6 mg/mL final, 200%) or BSA, recalcified, and clotted with TF (“Clot viscoelastometry”). Bars represent peak CEM (mean ± SD). (B-D) Recalcified human NPP spiked with fibrinogen or control was added to confluent cell monolayers. Fibrin polymerization was initiated in the presence of tPA; clotting and lysis were measured by turbidity at 405 nm. (B-D) Representative turbidity curves with human NPP and unstimulated HSVECs (B), SMCs (C), and TNF-α-HSVECs (D). Symbols are as follows: 3 mg/mL (○), 4.5 mg/mL (●), 6 mg/mL (■), and 7.5 mg/mL (♦) fibrinogen, final. (E) Time to peak turbidity and (F) peak turbidity (mean ± SD, n = 4), respectively. *P < .05 vs 3 mg/mL fibrinogen on HSVECs. #P < .05 vs 3 mg/mL within each cell type. (G) Recalcified murine PPP was spiked with human fibrinogen or HBS to achieve 2.4 mg/mL (○), 4.4 mg/mL (♦), or 6.4 mg/mL (▴) fibrinogen, final, diluted 1:3 in HBS, and clotting was initiated with TF (Innovin 1:30 000 final) and monitored by turbidity. Data are representative polymerization curves (n = 2). (H) Representative elastometry curves (n = 3) of human PRP and PPP prepared from CTI-inhibited whole blood, spiked with human fibrinogen (to 6 mg/mL, final) or BSA, recalcified, and clotted with TF in the presence of tPA (“Clot formation and lysis by turbidity”). The longer initiation phase of PRP clots versus PPP clots reflects the time to platelet activation.³¹