Post RP, biochemical recurrence can occur as far out as 15 years and PSA can fluctuate at very low levels during that entire time and yet you may never experience a recurrence.

The old definition of biochemical recurrence/failure was PSA of 0.2 (I believe two successive readings at or above that level.)

Since the advent of ultrasensitive tests, many now consider biochemical recurrence being an ultrasenstive result of 0.03 or greater.

Being a new user I can't post a link but google PMC4527538 for more info.

My post RP ultrasensitive tests at two large centers of excellence consider undetectable as anything less then 0.02 so the lab results will say "undetectable < 0.02". They don't bother displaying the actual number if it's less than 0.02 because studies have shown that readings below that are effectively unreliable indicators for any sort of actionalable decision making. I've read at least 6 studies following patients for years after treatment and how their uPSA results corresponded to whether they had biochemical recurrence or not. One study summed it up well in my opinion. It said a reading of 0.01 or less as far as its predictive ability is essentially the same as flipping a coin. So results in the .001 - 0.005 range are essentially meaningless except for the fact they are below 0.02 and therefore considered to be in the 'undetectable' range. PSA can be produced in tiny amounts by other things than Prostate Cancer cells is why. I'm almost wondering if you have your decimal point in the wrong place? As I've very surprised any lab would report any numbers below 0.01 since even below 0.02 is considered undetectable and inactionable and recurrence isn't considered until you hit 0.03 or about 75-300 times the levels you are referencing.

So in my opinion you are wasting your time being concerned about any PSA changes when results are < 0.02. No Urologist or Oncologist worth their salt should be recommending any sort of action based on PSA readings < 0.03. Keeping in mind they used to not take any action as long as PSA was < 0.2 (some medical professionals still may go by the higher 0.2 number but one study showed if you go by 0.03 it gives you on average 18 months lead time before you hit 0.2 so the salvage therapy has a much higher chance of being curative if performed after you hit 0.03 instead of the much higher 0.2)

A more interesting question to me is what PSA level is required to get insurance approval for salvage radiation treatment? I wonder if it's the older 0.2 level or if they now will approve at 0.03 due to recent studies regarding uPSA results being a strong predictor of recurrence. I suppose you could appeal a denial by referencing the latest studies. But I would bet it is virtually impossible to get approval of treatment at PSA < 0.03 or < 0.02. As these levels are not proof of recurrence so salvage treatment may not be necessary.

In addition PET PSMA scans are virtually useless at PSA < 0.5 in my opinion. They will miss areas containing cancer cells because the sensitivity rates of PSMA PET/CT according to PSA levels are 55-60% (0.2-0.5 ng/mL), 72-75% (0.5-1.0 ng/mL), 93% (1.0-2.0 ng/mL) and 97% (≥2.0 ng/mL)