i always go expunge first, countless first bloods when ambush would have lost them for me

Not a big fan of this item build, it seems really slow to start and twitch is a good early game ganker, with a good start he is practically unstoppable late game.

I hate to accuse people of things like this, but unless you go by multiple names, this build is stolen almost verbatim from Rydog's guide. Read from Summoner Spells down and you'll see.

http://www.leagueoflegends-wiki.com/lol/champions/twitch---the-plague-rat/twitch---aka-john-rambo-the-rage-inducing-steamloller

True indeed, didn't notice that when posting.
The mastery setup I posted is what I use with teemo and was fiddling with that for twitch too. If you use the masteries I posted you can take exhaust and a second spell by choice or when you use suggested spells loose the point in cripple and further down you can opt to take improved rally.