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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?

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I am pretty sure that there is no max or min, but that is kind of hard to test for. I think I once got it on the first hit, but it's been a while so I can't remember –  murgatroid99 Mar 28 '12 at 16:47
    
Nobody can really give a straight answer to that without extensive scientific research or requesting a copy of the game's source code from Mr. McMillen. –  lunboks Mar 28 '12 at 16:47
    
@lunboks A perfect answer (i.e. a specific max and min as provided in the code), no. Whether or not you can get it on the first try and whether it's possible to not get a blood bag after X uses definitely requires science, but it's apparently been done before (people referencing that average number of 8). –  Ian Pugsley Mar 28 '12 at 16:50
    
I can tell you that I've received it on the first go multiple times, and I've also once had to use it 32 times before I finally got the bag. So what I'm saying is it varies a lot. –  Mr Smooth Mar 28 '12 at 17:38
    
@MrSmooth Sounds like an answer I'd upvote! –  Ian Pugsley Mar 28 '12 at 17:51

2 Answers 2

up vote 11 down vote accepted

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.

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Blood machines are a nice way to convert a bomb into hearts but you may get full hearts or just the shadow (blue) hearts –  Dycker Mar 28 '12 at 18:42
    
@Dycker You can also sometimes only get coins, so it's a bit risky, but you'll mostly get hearts. –  Mr Smooth Mar 28 '12 at 18:59
    
I also don't recommend trying it if you do not have a high amount of hearts and might get down to only half a heart left before your 8th attempt at the blood machine without having a few cleared rooms with some more hearts in them. –  Wieland Mar 29 '12 at 8:30

One tactic is to first clear the whole floor, then visit the arcade or the blood machine (in the case that the blood machine stands alone in a room). There won't be any more fights on the current floor, which helps, though it's possible that if the machine doesn't oblige, the player could start the next floor in a weak state.

This works better if there are spare hearts lying around, or available in the shop. If coins are on hand, try any hobos, slot machines, or shell games. The Lovers tarot card, the Yum Heart, and Full Health pill are also all helpful. Lastly, if the My Little Unicorn or Gamekid items are on hand with a charge, use those to hit the machine multiple times without losing hearts.

With all that said, as others have observed the process can get tedious. As others have said, I've had the machine give up after one hit, or after several or many hits, or not at all. Push it as far as is comfortable.

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