Karen,  I don't have an answer to the amount of milk that must be
ingested in order to obtain an accurate PKU but here in Maryland it
seems to be moot point.  The same test covers many other inborn errors
of metabolism that are not affected by milk intake and must be done
prior to infant discharge.  The pediatricians do a repeat PKU (maybe
repeat of the other tests too, I don't know) in the office, usually at
the first visit.

Carla