investigations
Last reviewed 09/2021
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