Surprising that a Mayo doctor stated that < 0.1 is considered undetectable. My doctors consider anything measurable to be detectable, but less than 0.1 to be not actionable. Waiting for PSA to approach 0.20 before considered as a biological chemical recurrence and beginning next treatment.. However, measurements < 0.1 were used to track PSA doubling time and I was prepared for the next treatment as the PSA increased above 0.10