symptoms vary from person to person. because of the rarity of pku, many symptoms go undiagnosed or unrecognized. in addition, there are variations in the categories of pku individuals. there are untreated, early treated, late treated, those returning to diet, hyperphe, classical pku, uncontrolled, and highly controlled. for your average infant a doctor will diagnose pku with a quick blood test. their symptom will be high phe levels. if that infant goes undiagnosed for weeks or months, he/she may start to exhibit some or all of the following symptoms for uncontrolled pku. the symptoms include: