Phenylketonuria

In the US, and throughout the developed world, essentially all patients with PKU are diagnosed by newborn screening and follow-up diagnostic testing. This has been the case since the mid- to late 1960s. Patient populations in which PKU may be diagnosed on a clinical basis include adults with intellectual disabilities.

Newborn screening

This is performed by measuring phenylalanine (phe) in dried blood spotted on filter paper ≥24 hours after birth. If the phe level is above a cutoff established by the laboratory, the infant is identified as a candidate for further testing. If the phe level is repeated in plasma and found to be elevated with no evidence on testing for a defect in tetrahydrobiopterin (BH4) synthesis or recycling, the diagnosis of phenylalanine hydroxylase (PAH) deficiency is established. Ruling out a defect in BH4 synthesis or recycling is essential because therapy for these disorders is different from the treatment provided to patients with PAH deficiency. Once the diagnosis of PAH deficiency is confirmed, further subclassification as PKU or hyperphenylalaninemia will depend on the extent of the plasma phe elevation, both initially and as the infant's diet becomes more varied during the course of the first year of life.

It is important to remember that the newborn screening test can be normal in an infant with PKU if the test is obtained before 24 hours of age or if not feeding during this time. An additional test should be obtained no later than 7 days of age if the newborn screen was performed before 24 hours of age, if the newborn is markedly premature, or if the infant is sick and in intensive care.

History and physical exam

The prenatal and neonatal history of infants with PKU is typically unremarkable. A small proportion will have a previous sibling affected with PKU. Physical exam is normal.

The diagnosis of PKU should be considered in children adopted from a developing country who present with developmental delay, intellectual disability, or microcephaly. This can be ruled out by obtaining a plasma amino acid analysis. Consideration should be given to evaluating all foreign adoptees <3 years of age because the age at which symptoms are identified may vary.

Adults >40 years of age or those born outside of the country with intellectual disability should be evaluated for PKU. Although treatment will not reverse intellectual disability, it may result in improved behavior and reduction in psychiatric morbidity. Clues to the diagnosis in this population may include light pigmentation of eyes and hair, eczema, microcephaly, and seizures. Occasionally an abnormal "mousy" odor of the urine may be noted by caregivers.

Laboratory tests

Tests that should be performed to confirm the diagnosis in an infant with a positive newborn screening test for PKU include:

• Quantitative plasma amino acid analysis

• Blood dihydropteridine reductase (DHPR) assay

• Urine neopterin to biopterin ratio

• Mutation analysis of PAH gene.

Blood DHPR assay and urine neopterin to biopterin ratio tests will identify infants with defects in BH4 synthesis or recycling, who constitute <1% of those with a positive newborn screening test for PKU in the US, but a larger proportion in some parts of the world, such as Asia. It is essential to exclude inborn errors of BH4 synthesis or recycling at this stage because treatment for these disorders is different from treatment for PKU. These infants require neurotransmitter replacement because of the role of BH4 as a cofactor for enzymes other than PAH. Even with optimal therapy, defects in BH4 synthesis may have neurocognitive consequences. Plasma amino acid analysis will quantify the extent of the phe elevation and will distinguish true hyperphenylalaninemia, associated with PAH deficiency, from a generalized elevation of amino acid levels in plasma resulting from hepatic dysfunction or marked prematurity. The mutation analysis of PAH gene identifies specific mutations responsible for the disorder and so will confirm the diagnosis. If there are existing genetic test results, do not perform repeat testing unless there is uncertainty about the existing result, e.g., the result is inconsistent with the patient’s clinical presentation or the test methodology has changed.[5]​American College of Medical Genetics and Genomics. Five things physicians and patients should question. Choosing Wisely, an initiative of the ABIM Foundation. 2021 [internet publication]. https://web.archive.org/web/20230326143738/https://www.choosingwisely.org/societies/american-college-of-medical-genetics-and-genomics

Between 20% and 50% of all patients with PKU respond to treatment with BH4 with a significant reduction in the blood phe level.[6]Burton BK, Grange DK, Milanowski A, et al. The response of patients with phenylketonuria and elevated serum phenylalanine to treatment with oral sapropterin dihydrochloride (6R-tetrahydrobiopterin): a phase II, multicentre, open-label screening study. J Inherit Metab Dis. 2007 Oct;30(5):700-7. http://www.ncbi.nlm.nih.gov/pubmed/17846916?tool=bestpractice.com The BH4-responsiveness test is performed for treatment purposes to find the subgroup of patients with PAH deficiency who are BH4 responsive. Plasma or filter paper blood samples for phe levels are obtained before and at various intervals after giving oral BH4.