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:
Kamiya S, Hagimori M, Ogasawara M, Arakawa M.
Acta Haematol. 2010;124(4):218-24. Epub 2010 Nov 13.