Unfortunately the timing of when this test is done most of the time, increases the risk of a false negative -- meaning there is the possibility of NOT identifying a baby with PKU. Babies born to vegan mothers may even be at INCREASED risk of being falsely told their baby does not have PKU, because the vegan diet...
In fact the "treatment" for PKU is pretty nearly a vegan diet, except beans and nuts are avoided too. Babies are supplemented with a special mixture of vitamins and a small amount of protein which has had the phenylalanine removed. Below is how parents are taught to feed their child with PKU. Basically they can eat unlimited fruits and vegetables, and small amounts of grains, potatoes and peas. Meat, dairy and eggs are NOT allowed at all. (Keep reading past the image -- there's important stuff to follow...)
But lessor-known studies suggest that even carriers -- that is the 2% of the population who carry a single defective copy of the PKU gene, generally assumed to be fully capable of detoxing excess phenylalanine from their body, may experience elevations in phenylalanine levels when they consume a lot of protein, and this study showed they had increases from consuming Nutrasweet. We know from adults with PKU, that when their levels of phenylalanine begin to increase, their first symptoms are changes in mood and a decreased ability to focus and concentrate. It is possible that in those who carry only a single copy of the PKU gene, a higher protein diet may similarly impact them.
The interesting thing here however is that we now have examples of many people, consuming low-protein diets from infancy on (who have been diagnosed with PKU) living long full lives with notable physical and intellectual achievements. So the next time, someone tries to tell you that a vegan diet, which is usually lower in protein than one that includes daily consumption of meat, dairy and eggs is dangerously low in protein, you can point out how those with PKU seem to lead full lives while intentionally restricting their protein levels to much less than what most vegans typically consume.