I find this strange I was poisoned and use bloodletting. As you can see below it says the leeches were strong and consumed much of the blood but the poison length and strength loss remained the same and even the damage was higher after bloodletting.

*snip log*

It's one thing for the leeches themselves to work.
It's another as to how many of your affects they end up helping on, and if a given poison/disease resists it.