PSA alone is not a reliable indicator of anything.
One PSA result could be seen as a 'Red Flag', but can tell you little more than that.
Two PSA results will show a fall or rise, which can be useful (but isn't always).
Three (or more) PSA results can show the rate of rise (or fall), which can be very useful.
And in all cases, PSA results are only really useful when viewed alongside other results and findings.
PSA is part of the picture, not the whole thing.