WHO prefers all blood donations to be unpaid:
"Regular, unpaid voluntary donors are the mainstay of a safe and sustainable blood supply because they are less likely to lie about their health status. Evidence indicates that they are also more likely to keep themselves healthy."
Interesting. So the core question seems to be: "How much value is produced by healthy blood donors making decisions to donate without incentives, compared to blood that's "brought"".
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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