G6, in my professional life I manage a division that manufactures medical devices, including implants and delivery systems. As you can imagine the accuracy and repeatability of the measurement equipment used to verify finished parts is critical. In order to understand this in our environment we utilize tools such as "Gage R&R" and "MSA - Measurement System Analysis" to tell us how capable/repeatable our measurement equipment is. I've often wondered what the MSA is of the PSA test equipment used, and while I haven't found anything concrete, I did find this interesting blurb:
This is the most widely used test for detecting prostate cancer
today. It is simple to do. A small sample of blood is taken, usually from a vein
in the arm, and is tested for the presence of PSA (Prostate Specific Antigen).
This is an enzyme which was initially thought to be formed only by the prostate
gland - hence "prostate specific". This is not so and very small quantities of
the enzyme are produced by other glands - and even by women.
laboratory testing the blood will come back with a number, which usually
reflects the level of PSA in the blood in nanograms per millilitre (ng/ml). A
nanogram is one thousand millionth of a gram so the quantities measured are very
small. The method used to measure these minute amounts differs between the
manufacturers of the testing equipment and the results produced vary
considerably. Although all manufacturers agreed some years ago to calibrate
their equipment to produce comparable results (the Stanford Protocol), this
agreement is voluntary and is not always adhered to. It is best if you can have
all tests run by the same laboratory using the same equipment. Most laboratories
will only guarantee accuracy to within 80%.
Not sure if this helps at all, but I believe it adds something. Also in general and IMHO I don't see anything that would alarm me with your trend - so continued best to you!