I did some quick searching on the web a month or so ago, and I found that PSA readings near the limit of detection for a particular machine/device (in your case, 0.04) are not very repeatable and therefore very useful to judge if PSA is in a rising trend. You may want to search my earlier postings on what I said at the time, or do a search on "ultrasensitive PSA" yourself.
I had personally requested the "standard" PSA from my local urologist for these very concerns. However, what I got was the ultrasensitive so I guess I'll stay with it.
Age:45 (44 when diagnosed)
Father diagnosed and cured at age 52.
08/21/07: Diagnosed with T1c cancer
1 of 12 biopsy cores positive; 10% tissue
Gleason score: 3+3=6
PSA level prior to biopsy: 4.3 (velocity < 0.4ng/ml)
10/19/07: da Vinci prostatectomy by Dr. Vipul Patel
Difficult surgery due to prostate inflammation.
Both nerve bundles spared.
Spongy erections began within 24hrs of surgery!
10/24/07: Catheter out; down to 1 Serenity pad/day next day.
Final pathology: neg margins, no capsular penetration,
Gleason 3+3=6, 5% tumor involvement, multi-focal.
11/04/07 First usable erection with Cialis
11/22/07 Thanksgiving - Bye-bye, pads
01/17/08 First post-surgery PSA result: < 0.008 ng/ml