Well, 25 years or so ago in California, my wife I had to get a blood test to get married. As it turned out we were friends with the lab technician that drew our blood for that test.
I think it would have been very easy to have had the test "fixed". The blood was taken in the local hospital lab area. That would have been the place to "introduce a variable" into the test. This test is really for the benefit of the people getting married and the controls were not very tight at all.
This was in a small town. It may be more complicated in a big city.
Bob
