Hi Karen, According to "Understanding Lab Work in the Childbearing Year" by Anne Frye, the PKU test should be done after the milk has been in for 48 hours to allow for sufficient buildup of protein. This is how it is generally done in our (Boston, Mass.) homebirth community. At the hospitals it is done on the day of discharge, which is often the day after the birth. There was an article in the Boston Globe saying that one of the problems with 24 hour discharge is that the PKU tests wouldn't be accurate (as opposed to when they were done before 48 hour discharge.) But it is my understanding that the test wasn't accurate at 48 hours either. Hope this helps, Julie Brill, in Bedford, MA USA [log in to unmask]