I happen to administer a lot of blood to my patients, so let me answer some of the factual questions.

1. The way they calculate "up to 3 lives" is in the most trivial way: blood you donate is fractionated into red cells, plasma, and platelets. Each of those may go to a different recipient.

2. All blood administered to patients comes from voluntary, uncompensated donations. Plasma used in research studies may be compensated, but may not be transfused. This is the most important factor keeping our blood supply safe, and is far more effective than laboratory testing alone.

3. Given that blood banks need to keep a sufficient store of blood available of each type, rarer blood types are generally in greater need than, say, A After all, a larger proportion of blood of those types must be discarded. O blood is obviously highly useful in trauma situations, and is therefore in high demand as well.

4. The distribution of donors' and recipients' blood types should not be assumed to be equal: people with blood type A are significantly more likely to donate than people with blood type B. This exacerbates the discrepancies due to point 3.

5. The number of lives saved can be calculated in two ways:

a. the feel-good way. Every time a physician gives a unit of blood to a patient e does so believing it is a life-saving procedure. So if 3 units are given the patient's life was saved 3 times in rapid succession. (You have to be willing to save a life multiple times, because that's the analysis we're using for the rest of this discussion: multiple mosquito nets saved the same kid's life multiple times over his lifetime; that same kid was then saved by anti-diarrheal treatments; etc. The same analysis belongs here). Now, we subtract the number of patients who die, but that's a small number. So 26 million transfusions/16 million donations = 1.6 lives saved per donation.

b. the marginal way. Donations are currently sufficient for usage; we benefit in three ways from more donations. First, we can be slightly more profligate with trauma patients who have a low survival chance; this saves a minimal number of lives. Second, fresher blood is associated with better outcomes than older blood; the extent of this effect is unknown but is an area of current research interest. The calculation would have to look at the likelihood that your donation reduced the average shelf age of the blood being administered times the survival improvement from the fresher blood. Third, blood from multiparous women is associated with ARDS; an increase in donation would allow us to stop using it.