After RP, PSA diminishes in the body in a mathematical model similar to radioactive decay...the half-life of PSA varies person-to-person, but has been shown repeatedly to be in the 2-3 day range. The residual value in any one person's blood is a function of how high it was to start with (before surgery), that person's actual rate of decay (again, almost always between 2-3 days), and whether there is another source of PSA in their body (like progressed prostate cancer, or benign sources from nerve sparing surgery).
So in your case, you could estimate your PSA on the day of surgery at 4.0 ng/mL, and assume a slow decay rate of 3-days:
- after 3 days, your PSA would equal 2 ng/mL (or half the prior amount)
- after 6 days, 1 ng/mL
- after 9 days, 0.5 ng/mL
- after 12 days, 0.25 ng/mL
- after 15 days, 0.125 ng/mL
- after 18 days, 0.0625 ng/mL (below the threshold for "standard" PSA test)
- after 21 days, 0.032 ng/mL
- after 24 days, 0.016 ng/mL
- after 27 days, 0.008 ng/mL (below the threshold of an "ultrasensitive" PSA test with 0.01 lower detection limits)
So you can see, after 4 weeks, someone with a low pre-surgery PSA of 4.0 ng/ML would most likely be below detection limits of any PSA test. You can see that someone with pre-surgery PSA of 8.0 ng/mL would decay to the same undetectable level in just 3 more days...and so on and so forth.
Just to be sure, the common rule of thumb is to give a little more "margin" to these numbers. 6-weeks is common.
Post Edited (Casey59) : 4/11/2011 2:39:06 PM (GMT-6)