My point was not clear.
I have never seen anything on a phosphate test myself. Every other test I have seen the good and the bad, but not a phosphate test. Even when I was having pest algae problems. Even when phosphates are a problem they can be used up before they are tested for. The best test can say zero and you can still have a phosphate problem. So in my experience, you should know where phosphates come from and know how to remove them, but I, myself, have not found need to test for them.