Interesting question, Mel. I'm pretty sure that the term "undetectable", as it applies to most studies, refers to the standard <0.1 ng/mL. You are, however, exactly right, in noting that with the newer ultrasensitive assays, any value reported, for example, 0.02, is indeed detectable. The question remains, is a PSA value less than 0.1 meaningful? Do you start further treatment based on this, or based on an upward trend?
A lot, undoubtedly, is based on an individual case and the pathology report. And also on the medical philosophy of the physician and patient. Is it better to treat on the possibility of further increase in PSA, or is it better to wait and see. This is perhaps the same kind of decision process behind how to treat an early cancer like mine. Is it indolent? Should I wait and see (Active Surveillance), should I treat with surgery, with radiation, or other treatment (including experimental treatments) ?
One thing is certain: prostate cancer is protean, perplexing disease.