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Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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In classic phenylketonuria:

  • blood phenylalanine concentration - normal at birth but rises thereafter (more rapidly in males). Blood screening for phenylalanine identifies most cases at the age of 1 week; urinary analysis at this stage is less accurate. Concentrations of many other amino acids in the blood are generally significantly reduced

  • phenylalanine challenge - this is contraindicated in patients with known phenylketonuria - if this is undertaken then plasma tyrosine does not rise

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