1. expunge is a must for your first skill, the problem with ambush is that you don't need it right away, that's why we have bushes(not to mention you cant stay invisible for long at level 1). most of the time during first contact the opponent gets away with little health, this is the time for expunge.
2. life steal is not needed but it helps a lot. getting an item like executioner's call early on helps with lifesteal almost the whole game.
3. flash is also has greater uses than cleanse, simply being you could get stunned yes, but your more than likely to get slowed, fear, etc., and if you get slowed flash behind an object. this will get you further away from the enemies, and by the time they catch up to you the slow will be worn off. Flash also helps for chasing and running from champions. The little distance you gain from flash can give you enough time to use ambush.

Further more, i cant agree with low ambush in the beginning.
Whats with the armor and magic resist runes? you need critical chance/damage or attack speed
Debilitating Poison should not be gotten that early, its the least needed skill that is if you had good as or crit in runes.
Again with your masteries why do you need magic resist and armor. twitch is squishy hes an assassin after all, your wasting runes and masteries by making him more resistant to attacks but less deadly.

i tried it, it's nice

Thanks for posting, well I really can't agree on you.
Ambush is very usefull at lvl1 so you can spy on your lane and understand what enemies are there before they even realize there's a Twitch around there.

Really, Ambush needs to be a lvl1, if you really like to have the first blood grab Exhaust instead of Cleanse, go invisible in the bush if there's only 1 enemy there call your lane mate, exhaust him and it'll be so easy to kill him!

But never level expunge at lvl1.
Ambush is the main ability on Twitch.

Show me some results with my build if you have ;)

Thanks.

this build is really nice. The only thing different that i do is I level expunge first, then ambush at level 2.

I am only level 20 so i cant say i have all the runes but without them all i still usually get a lot of kills, unless of course i get focused.
+1

Lifesteal is waste of money, if they focus on you use your own skill combined with your spells (cleanse and flash) can get you out of any situation...
Lifesteal is good at beginning so you can keep on lane, but actually your point is not to kill minions but yes to kill low-health enemies, as soon you have the Mobility Boots you can start ganking even before lvl6.

If they focus on you (you should be far using the 1000 range that ultimate provides), you have enough time to apply the slow and run away.

Note: I'm not teaching, this is just how I think and how I play, give me your opinion if and HOW you use lifesteal on twitch.

:)

its a good build just wonder why you dont get any life steal for survivability it helps alot espically when they focus on you.

Pretty nice dude. I used to make a huge victory with twitch. Rock it baby!!!!!

Nice :)