text: what is a pku done on the babay and what happens if it comes back bad? a friend just had a babay and was told they needed to run some more blood tests because the pku came back bad. what does this mean pku, or phenylketonuria, is a genetic disorder where the person lacks an enzyme to break down a certain amino acid called phenylalanine.  in children with pku, phenylalanine builds up in their bodies and can cause problems.  it used to be that it wasn't diagnosed until symptoms showed up, at which point it was too late to prevent a lot of the damage from the amino acid build-up.  children with pku would typically be signficantly mentally retarded, and have other developmental problems as well.  now, however, all children are tested for pku at birth, because a really cool scientist named robert guthrie decided not to patent the test, so it would be available to everyone at a reasonable cost.  when a baby is diagnosed with pku at birth, they are put on a special diet that is low in phenylalanine so that they will not have any symptoms later on.  as long as they stick to their special diet, children with pku can develop perfectly normally, and show no signs of mental retardation or other problems associated with untreated pku.  so basically, what it means that the test came back postivie (i.e. "bad") is that there's a good chance that the baby has pku and will only be allowed to eat certain foods (like how people with diabetes can't eat things with a lot of sugar), but since they caught it so early, he/she will be fine as long as his/her parents follow the doctor's instructions carefully.