Blood bags are used to fully restore a players Blood in the form of a blood transfusion, which can only be performed on another player. Originally blood bags would instantly heal a player, but now heal over 12 seconds from the point of administration (1000 blood per second). They can be found around Hospitals in medical boxes and on the ground, as well as in the medical boxes that spawn near crashed helicopters.
In 1.8.1, the player can create their own blood-bags that they can apply to themselves with no need for another player. These are created by first finding a Empty Blood Bag, usually found in medical tents and hospitals, right clicking on them and clicking "Fill Blood Bag". This type of blood bag only restores 4000 blood, whereas the other types will restore 12000, putting the player at max health. The blood bags found in hospitals cannot be used by yourself.
Currently there are AB-, AB+, A-, A+, B-, B+, O- and O+ Blood Bags in game.
You must know the blood type of the recipient before you give a blood-bag, unless it is of type O-, which is compatible with everyone. If you give an incorrect blood type transfusion, the patient will be unconscious for an extended amount of time. Find a Blood Test Kit in hospitals and medical tents, use it in your inventory and a screen will open where you can test the blood. Double-click on the water (right) and put it in each section, followed by a sample of your blood. Then double-click on the "How to read results" sheet at the back and compare how your tests look to the results page. You now know your blood type.
After a blood transfusion, the player who administered the transfusion will gain 250 Humanity points.
If the player applying the transfusion breaks away from the transfusion animation or the recipient moves out of range, the transfer stops and the remaining amount is lost along with the bloodbag in use.