Comparison of present to previous test results would probably be quite futile given almost all the kit nowadays has a detachable IEC mains lead. Variations would be bound to occur on mains side results due to swapping of leads. The danger is then that a considerate techie will waste precious time investigating something insignificant, causing time pressures elswhere on significant risks. Also since it's the mains lead itself that's the point of failure, tracking them would be more usefull than fretting over 35 microamps higher earth leakage on the latest test.
Several years ago I tried serial numbering IEC leads and had to drop it because it took too long! The only way it could be made practical would be to instigate it as part of the acceptance procedure so numbers are phased in. Haven't tried that yet.
As regards patient leakage. I have never seen a "failing" isolation circuit. It either works or (typicaly post defib pulse) doesn't. In which case you will also have no patient signal on that parameter. I haven't had an isolation breakdown in the last 18 years the kit is so well engineered these days.
Marc