Comparative diagnostic value of phenyla-lanine challenge and phenylalanine hydroxylase activity in phenylketonuria

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Abstract
Serum Phe concentrations during and following Phe challenges and liver phenylalanine hydroxylase (PH) activity were compared in 13 phenylketonuric (PKU) patients. These patients were separated into 2 groups: 8 patients with no detectable PH activity (PH.degree.) and 5 patients with residual PH activity (PH-) ranging from 9-24% of the activity obtained in 10 non-PKU subjects. The rise in serum Phe concentration during 3 days of oral loading did not differentiate the 2 groups. The difference in serum Phe concentration of the PH.degree. and PH- groups reached statistical significance at 24 h postloading (P < 0.01). Combined results from multiple measurements during the oral challenge, namely serum Phe concentration after termination of loading, serum Phe clearance rate, post-loading Phe tolerance index and urinary metabolite excretion, make a better indicator for predicting residual PH activity for the majority of PKU subjects than peak Phe concentrations during Phe challenge.