|It is a misnomer that your baby has to be eating milk for the test to be accurate.|
I just wanted to say that I'm not sure about the accuracy of this information. I think Pam is just about always right but my daughter has PKU and we have been told nearly the opposite.
There is a small shift in phenylalanine levels before the 24-48 hours of good feeds, and most babies who have PKU and are tested during that small shift will still get identified. However, there are a handful that won't. So, if you are only going to to the test once, please do it after the baby has been eating well for a few days.
In the early days after her diagnosis, when we checked my other children, we actually did a "phe challenge" where we fed them as many complete proteins as we could, waited a certain amount of time & then had the labs drawn.
Other metabolic disorders may not need the 24-48 hours of good feeds, and since I'm not an expert on those, I won't comment. But, the PKU test (specifically) will give the most accurate results when feeding is well established.