Nattokinase has been shown to prevent the formation of clots in vitro, that means in a test-tube.  However, the question remained, can it do the same thing in vivo (inside a living organism).

In an article in Acta Haematologica, Japanese researchers from the Nagasaki International University injected nattokinase into the tails of rats and found that it exhibited significant anti-clotting effects.

They concluded that nattokinase does have anti-thromobtic effects inside the living organism.

Source:

In vivo evaluation method of the effect of nattokinase on carrageenan-induced tail thrombosis in a rat model.