MMP9 increases in venous compared with arterial thrombus. The activity of MMP9 in proteins extracted from venous thrombus (DVT or PE) compared with arterial thrombus (acute stroke) using a chromogenic substrate. A MMP9-specific chromogenic substrate confirms that MMP9 activity is markedly higher in venous than arterial thrombi (61 ± 9 ng/mL per μg protein vs 25 ± 6 ng/mL per μg protein). ∗∗P = .0051 (MWU).