This came up for me this month; I had PSA tests from 2 different sources. In my current treatment PSA varies quite rapidly. Before accounting for the difference between the two tests, I thought the score had dropped 1 point. In reality, it had dropped 8 points - a big difference!
This information comes from Beckman Coulter, who produce a standard PSA test. The URL links to their website, to an article called "The PSA Gap."
[quote]"Same blood. Two PSA tests. Different results. "That amount of difference is not due to biological variation or assay imprecision," says Bernard Cook, lead scientific advisor for Beckman Coulter.
"How can two PSA tests result in such significantly different readings? The problem stems from the way PSA tests have been standardized. "There are two different standards or ways to calibrate PSA tests," explains Dr. Cook. "And it's become evident that they don't deliver equivalent results."
"In fact, one method consistently delivers results that are roughly 22 percent lower than another method. [ end quote]
So the Siemens test, which conforms to the WHO standard, produces results that are 22% lower than the Beckman Coulter test, which is based on an equimolar standard.
Just be careful that your treatment isn't influenced by this trait. Many doctors are not aware of the differences.