I have a similar question my son get his levels always after a bottle of phenyl-free and not breast milk and the nutritionist has told me how older children fast before getting levels done. I do not totally understand this and am not finding the info online. Would the level be much higher after a breastmilk feed and does anyone know how long the level stays at a level before changing? Is it like a constant up and down with his levels from feed to feed.?