I donated blood regularly thru the American Red Cross before my blood clots and ET diagnosis in 2001. I was told for years after pre-donation screening that my hgb and hct were high, and that those could be indicative of a serious blood disorder. This did not deter them from taking and using my blood, tho what we have is not contagious. I suspect the regular phlebotomies kept my ET under control for a long time.

No, unless you have a diagnosis of MPN, they do not screen. And it would be so easy to do it, just look at blood counts. Even if someone without MPN would have the thrombocytes or leukocytes high, they should not be allowed to give blood, as that indicates they have some sort of infection at the time of blood donation. And given that for many of us it takes years from first increased blood counts to diagnosis, this means that people who had undiagnosed MPN could have easily donated blood.
It would make a world of difference in our diagnosis if there would be some screening for blood counts. JAK2 mutation was found to be present in ~3% of a population sample and gave increased blood counts in ~3% of these ~3% with the gene. This means MPNs might be rare, but not extremely rare as it is thought, and that many people are left undiagnosed and untreated.

No, they do not. I donated blood before I knew I had a MPN. though that was many years ago.

I doubt it since it's so rare.

Janet good question. I know that you can’t donate if you have an MPN. Linda