Hunh. I have tried for years to get consistent result from Hanna PO4 kit and have given up. Obviously, since so many others swear by the test, the fault is mine, but regardless, I can't coax consistent results from the thing. Salifert, Red Sea, and Elos kits all read zero whenever I use them, and so I have discounted them as useless.
The ONLY PO4 kit that given me consistent, plausible result is the Hach kit.
http://www.hach.com/phosphorus-orthophosphate-reactive-test-kit-model-po-19/product?id=7640214958
It ain't cheap, but it works for me.
Mike