Discrimination of phenylketonurics from persistent hyperphenylalaninemia patients using a simple phenylalanine loading test.

A simple, oral phenylalanine loading test was developed in order to discriminate between controls, phenylketonuria and persistent hyperphenylalaninemia patients. Only three capillary blood specimens for quantitation of phenylalanine and tyrosine over a short duration (90 min) were required. Using stepwise multivariate discriminant analysis, accurate classification was achieved for 27 controls, 12 patients with phenylketonuria and 6 with persistent hyperphenylalaninemia. This loading test and analysis is both simpler and less expensive than those previously described.