I think that depends on your state - some states recommend waiting until 48 hours. It can also depend on the type of testing that they utilize. Another determining factor would be how well the baby was eating, etc? The key thing to remember is that this test tests for more than just PKU.
Personally as someone who now has three children with PKU - yes I would test again just to make sure, but that is me - had I never had kids with PKU I might answer differently, but considering I do I would say to go for it