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In a child with dihydropteridine reductase deficiency (McKusick 26163) none of four separate oral tetrahydrobiopterin loading tests resulted in a decrease of the serum phenylalanine concentration. This test cannot be recommended for screening hyperphenylalaninaemic patients for dihydropteridine reductase deficiency.
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Lipson et al. (1983) studied this question.
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