So, I've been wondering about this...so...Kaname kills Shizuka and drinks her blood...later in the story, when Zero is in locked away because he is falling to Level E as a result of Shizuka, his master, being killed, Zero drinks Kaname's blood. When Shizuka was in Maria's body, she told Yuuki that Zero just had to drink her blood, and he would be saved from becoming Level E. Now, Zero drank Kaname's blood after Kaname drank Shizuka's blood...so...if Kaname had Shizuka's blood still in his system...would it not make sense that that would keep Zero alive?
It's been buging me for a really long time...

Yea i know it's been bugging me too but i really don't know if the blood will prevent Zero from degenerating into a Level E vampire. I think it may just slow down the process. Seriously that would be sad if it really happened but there's a little hope.......

i tink its cause zero drank the mix of kaname and shisuka's blood sooo...

I KNOW!!! It'd be so boring without Zero around!!! I'm a hardcore Kaname fan...but without Zero I'd cry!!! =P