To the best of my knowledge there is no harm in having another pull of blood and another test to confirm whether the < 0.02 or 0.02 reading occurs again. I'm not the expert on why PSA would fluctuate after the prostate is removed, but I'm sure there are reasons, as we learn more about this all the time.
There is no harm in increasing the frequency of your testing, but I would encourage in the interim time period of testing to self-educate yourself on biochemical recurrence and the various treatment methods that exist (salvage radiation therapy, androgen deprivation therapy, etc.). No one "wants" to join the club of fighting this cancer after taking the radical step of having surgery, but early-detection equates to most-options, so time is on your side if you are well-equipped with information.
The path to approval of treatments varies from what I have seen. I had to do a sequence of bone scan to MRI to CT scan and then I was approved for a PSMA PET scan, fortunately I did that sequence in 5 weeks, but my gut told me to start with the PSMA PET, but the insurance company wanted us to walk our way there. I suspect a similar protocol might exist for salvage radiation.
Keep the faith and enjoy each day, life is fragile enough without having to deal with cancer.
I've read there can be multiple causes for ultrasensitive PSA fluctuations especially for those getting results to 3 decimals! But it seems accuracy better than 0.005 is questionable. From what I just read uPSAs are considered reliable to 0.01. So could mean a 0.015 could be anywhere in between 0.01 to 0.02. It's crazy to think the tests used to only go one decimal and still due for screen purposes. Boy, opting for 3 digit tests seems like a recipe for some serious anxiety. Which I can then see why many places consider "undetectable" as < 0.02. Giving numbers lower than that is splitting hairs in my opinion.
The problem is now I have to live in doubt until my next test. I think I'd prefer to go in a day later for another draw just to rule out lab error which is also a possible factor. That would give me chance of more peace of mind until the next test 3 months later (although someone told me when they hit 0.02 their doctor recommended going in in 2 months rather than 3).
My tests are free being on Platinum insurance plan.