A small change in inhibitory kinetics may explain the thrombosis associated with combined FVL and TFPIα haploinsufficiency. TFPIα inhibition of prothrombinase containing FXa-FVa (red) is reproduced from Wood et al.¹⁰  TFPIα inhibition of prothrombinase containing FXa-activated FVL (blue) is extrapolated, assuming a 1.7-fold increase in IC_50. With FV, 50% inhibition is achieved by 0.9 nM TFPIα (i; green dashed line). This same concentration of TFPIα would be expected to inhibit ∼30% of prothrombinase containing FVL (ii; purple dashed line), and a 50% reduction in TFPIα would result in ∼10% prothrombinase inhibition (iii; orange dashed line).