I haven't seen a good primer on PSA - it can be quite complicated.
“Zero isn’t Zero” - there are two aspects of this, the lowest that any test can measure, and that there are other sources of PSA in the body, albeit tiny, be even women have measurable PSA levels on the most sensitive tests. You'll see references to "undetectable" but I consider this term useless, because what's detectable on one tester won't be on another using a different model or assay, so undetectable has no uniform meaning. Each tester/assay has a minimum value it can measure. Some common ones are:
If you are below the minimum for that tester/assay combination, you will be given <0.1, <0.01, or <0.003 (for the three examples above).
The lab at my local hospital measures down to 0.01.
The lab at my radiotherapy hospital measures down to 0.1, but that's not good enough for prostatectomy patients and their samples have to be sent to one of the central London hospitals which does down to 0.003 although 0.01 is regarded as good enough in this case.
Another hospital linked with a support group I run does prostatectomy patients down to 0.003, and all others down to 0.1.
Different cancer cell mutations produce different amounts of excess PSA. You might get a rough idea in your case from what your initial PSA at diagnosis was, and the extent of the cancer. If you've had prostate cancer a long time, you will probably have multiple different mutations among the cancer cells, and their PSA levels may be different. About 15% of prostate cancers don't produce any extra PSA, and those can be more difficult to pick up in the early stages.
What tends to be more useful is to look at the rate of change of PSA than the absolute numbers, when monitoring for progression. You're looking for stable numbers, rather than increasing numbers, but to be low is good too.
For protatectomy, ideally your surgeon will want you to be well below 0.1. Three consecutive increases above 0.1 or hitting 0.2 are the usual triggers for further investigation, although the investigation doesn't necessarily happen at that point - often they need higher levels to initiate some types of investigation.
However, this isn't driven entirely by PSA. Histology factors in too. Sometimes they will know from the histology they left some behind, and if this is cancerous and aggressive, they may initiate further treatment sooner. Sometimes some prostate cells are left which are not thought to be cancerous (nerve sparing can do this), and these will give a higher background PSA level, but that is expected to remain stable and not increase.
The high accuracy PSA measurements can cause patient anxiety when the least significant digit bounces around, but an oncologist will understand this. These can be very useful when working out things like doubling times. One prostatectomy patient I spoke with who was being incorrectly given the low accuracy ones when his PSA was rising meant they couldn't immediately work out his double rate, which would normally have been important in selecting followup treatment. This was regarded as a medical mistake.
PSA is a much better tool for monitoring treatments than it is for initial diagnosis. It was indeed originally used for this, before anyone started using it for initial screening.
Wishing you the best result and all the best for 2021.