The Blood Donation Machine allows you to trade half a heart pre-Womb or a whole heart in or after the Womb for some amount of money. Additionally, there is a chance that the machine will explode and give you a Blood Bag, which gives you another heart and heals you five hearts (unless you're playing as ???).
I've read that the average number of usages to obtain a Blood Bag is around eight. I've never made it as high as eight personally, but it's supposedly possible to go well above that.
Is there a maximum or minimum number of times before a Blood Bag will drop? If I have a certain number of hearts, am I guaranteed a Blood Bag? Can it drop on the first use of the machine?
Best Answer
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.