Detection of heterozygotes for phenylketonuria. Total body phenylalanine clearance and concentrations of phenylalanine and tyrosine in the plasms of fasting subjects compared.

Two tests have been compared for detection of heterozygotes for phenylketonuria, one based on determination of plasma phenylalanine and tyrosine concentrations in fasting individuals and the other on kinetic evaluation of the plasma elimination curve after intravenous loading with L-phenylalanine. The plasma elimination curve was biexponential and the kinetics were evaluated according to the two-compartment model. The constant, beta, expressing the rate of elimination from plasma at pseudo-equilibrium, the rate constant for the elimination from the central compartment, and the total body clearance were determined. Of these three, total body clearance, which on the average was reduced by 32% in the phenylketonuric heterozygotes, showed the best discriminatory ability, but was not better than the information on concentrations of phenylalanine and tyrosine in detecting heterozygotes for phenylketonuria.