The goal of this feasibility study clinical trial is to compare the accuracy of PheCheck™ for the rapid quantitative detection of phenylalanine (Phe) with the gold standard (HPLC amino acid analyzer), in patients with PKU.

The main aims are:

• Evaluate the accuracy of PheCheck as compared to the gold standard
• Evaluate ease of use by lay participants

The primary objective of this feasibility study is to evaluate the accuracy of the analyte concentration of PheCheck™ for the rapid, quantitative detection of phenylalanine (Phe) from capillary blood compared to the gold standard (HPLC amino acid analyzer) which quantitatively measures Phe in venous blood.

The study will enroll subjects into one study cohort. It will include patients 10 years or older, monitored for Phenylketonuria (PKU), hyperphenylalaninemia or PKU during pregnancy.

Inclusion Criteria:

• 10 years of age or older
• Being monitored for Phenylketonuria (PKU), hyperphenylalaninemia or for PKU during pregnancy Signed informed consent

Exclusion Criteria:

• Younger than 10 years of age
• Not being monitored for Phenylketonuria (PKU), hyperphenylalaninemia or for PKU during pregnancy
• Lack of signed informed consent
• Previous enrollment in the study and has completed study visit 1 and 2

Patients ≥ 10 years who are undergoing monitoring for Phenylketonuria (PKU)