When should I use the Blood Donation Machine?

The amount of times you'll need to use the machine varies a lot.

I've received the bag on the first attempt multiple times, but I've also once had to use the machine 32 times before the bag finally appeared.

Most of the time it's worth it to at least give it a go, as you'll generally find it around 8 attempts, which will fully heal the hearts you spent.

The only time I don't go for the bag is if I'm already low on hearts and I think I'm going to need them.

Also, another note, it's generally worth it to give it at least 2-4 attempts because even if you don't get the bag, you can blow up the machine and possibly get a heart or two.

In the Wrath of the Lamb DLC, the blood bank is definitely a harder choice on whether to use it or not, as now instead of getting the blood bag, you'll sometimes get the portable IV bag, which isn't so good.

Now it's best to just use it if there are spare hearts lying around the level that you can pick up, or you need the extra money.