Criteria
NIH consensus development statement criteria[1]
Classical PKU: a disorder associated with high levels of blood phenylalanine (phe) (typically >20 mg/dL) and a complete absence or profound deficiency of phenylalanine hydroxylase (PAH) activity.[1]
Non-PKU hyperphenylalaninemia: a lower elevation of blood phe.
Exclusion of tetrahydrobiopterin (BH4) deficiency states is not specifically mentioned in definition but assumed.
European classification[11]
Classical PKU: complete or near-complete deficiency of hepatic PAH activity accompanied in the patient by a dietary phe tolerance of <250 to 350 mg/day.
Moderate PKU: characterized by a dietary phe tolerance of 350 to 400 mg/day.
Mild PKU: characterized by a dietary phe tolerance of 400 to 600 mg/day.
Mild hyperphenylalaninemia: a condition in which plasma phe levels are <10 mg/dL on a normal diet.
Common practice in the US
Classical PKU: blood phe on unrestricted diet >20 mg/dL.
Non-PKU hyperphenylalaninemia:
Mild-to-moderate or nonclassical PKU: blood phe on unrestricted diet above the threshold for triggering intervention but <20 mg/dL
Benign hyperphenylalaninemia: blood phe on unrestricted diet above normal but below the threshold for triggering intervention (typically the threshold is 6 mg/dL).
Patients who have hyperphenylalaninemia resulting from rare defects in tetrahydrobiopterin synthesis or recycling are excluded from this definition.
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