system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When my blood donation reacted during a test What is it that's reacting and what does this mean? Also why can't my blood donation be used?

So I received a letter from my local blood bank and after a few rejected donations I was told that my blood donations continue to give a "reactive result" They perform these tests on every donation and I won't be eligible to donate any more.

Apparently these reactions are quite common and nothing to be worried about.

What exactly do they test for? (apart from the obvious diseases and infections) and what is it that gives this reactive result?
gpt
They test blood for all diseases that could be transmitted through a blood transfusion.

Some people have antibodies that cross react (don't have the disease, just have an antibody that binds to the site that causes the test to be reactive)

The first tests are incredibly sensitive, but not very specific, so when they get a reactive, they do a confirmatory test, which is very specific, but very expensive, which is why they don't do them on the whole lot. Confirmatory tests also take a long time to complete.

If the confirmatory test is negative, then you don't have the disease, but the FDA says that the blood still cannot be used.