Karen,
In our state we are required to do a PKU test on all babies prior to discharge even though the PKU portion is not yet considered valid.  Several metabolic disorders are screened for in this test other than PKU and these tests are valid.  The parents are
asked to bring baby back after 5 days to have the test repeated.  Babies are born with enzyme "blueprints" but don't have a quantity of enzymes until an exposure to the thing it needs to break down.  A baby tested for PKU at only a few hours old has not
yet made enough PKU to be detactable.  By waiting a few days and repeating the
test you eliminate the false positives.
I hope this message isn't as muddled as I feel.

Becky B.
Moscow, Idaho