Carrier screening for phenylketonuria: comparison of two discriminant analysis procedures.

Absence of a convenient, direct enzyme assay for detecting phenylketonuria (PKU) heterozygotes has resulted in continued effort to develop an accurate and reliable procedure to discriminate the heterozygous individual from the homozygous normal. Our study compares two statistical procedures that combine the semifasting plasma phenylalanine and tyrosine concentrations with the individuals' prior probability of being a heterozygous carrier in order to discriminate carriers from noncarriers. The results of this comparison indicate that the quadratic discriminant function is superior to the linear discriminant function as a method of carrier testing both in theory and in practice. An interactive computer system is described that facilitates the clinical utilization of the quadratic discriminant function.