Standardized loading test with protein for the differentiation of phenylketonuria from hyperphenylalaninaemia

Eighty-nine protein-loading tests were performed in 80 patients with an increased blood phenylalanine concentration. The amount of protein corresponding to 0.18 g phenylalanine/kg body weight per day was calculated and given for 3 days, during which time eight phenylalanine and eight tyrosine blood concentrations were determined. Nine numerical evaluations were checked for discrimination between phenylketonuria and hyperphenylalaninaemia. The two most powerful discriminants were the phenylalanine blood concentration after 72 hours (sample taken in the morning of the 4th day after the beginning of the load) with a limit value of 15 mg/dl (0.91 mmol/l), and the mean value of the 5th to 8th phenylalanine values, also with a limit value of 15 mg/dl. In only eight of the 80 patients would the final diagnosis not have been established by the loading test.