system
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human
ELI5: Why blood bag that isn't completely filled up cannot be used?

So I remember a friend having this really large bruise on her elbow pit. Apparently she donated blood but the nurse said her vessel popped so the needle had to be removed. She was informed the blood can't be used for donation and would have to be discarded because it wasn't completely filled up. Even tho it wasn't full, the bag had lines to measure the amount of blood, so why?
gpt
Blood bags are pre-filled with chemicals to prevent clots and extend the shelf life (most commonly CPDA). These also affect the acidity of the blood. The amount is calculated assuming the bag is filled to a certain volume.

These remain in the blood when it is transfused, so if you used a bunch of partial bags instead to transfuse someone, you would be transfusing them with a bunch of excess anticoagulatant and also messing with their pH. Since they are presumably sick enough or bleeding enough to need a transfusion, they might not be able to handle this.