FVL reduces the TFPI-dependent activation threshold for FXa-initiated fibrin formation. Fibrin formation was initiated in FV plasma (red) or FVL plasma (blue) by the addition of (A) 5 pM or (B) 0.5 pM FXa, 4 μM PC:PE:PS, and 5 mM CaCl₂, and monitored at 405 nm. Experiments were performed in the absence (solid lines) or presence of anti-K2 (long dashed lines) or anti-K3C (short dashed line). Experiments were performed using 8 each of FV and FVL donors. Shown are representative fibrin formation curves from a single FV donor and a single FVL donor. OD, optical density.