D&C, I have had some small but erratic variations to my PSA results. Even though I don't have a prostate I avoid sexual activity etc. I and one of Lyn's men have had our blood samples taken at the same time put in two separate viles and both tested , mine in the same lab, the difference was 0.01. So machine tolerance etc can account for small variations.
I now have mine done at the same time of day at the same regional health centre, usually around half an hour before the samples are transferred to the lab. I did wonder if the samples taken at the hospital and tested within an hour would give a different result to tests I had done early in the morning at my regional health centre and then transferred to the lab by van 5 hours later.
In conversations with many medics I don't think any of us had had a definitive answer to variations, and I don't suppose small variations are as significant to our medics as the overall long term pattern. My oncologist said it is the patients blood sample that is unreliable not the testing equipment. I wonder if the half life of PSA is the same in all men.