Hyperphenylalaninemia Workup

Updated: May 18, 2017
  • Author: Eric T Rush, MD, FAAP; Chief Editor: Luis O Rohena, MD, PhD, FAAP, FACMG  more...
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Laboratory Studies

Screening for hyperphenylalaninemia includes the following:

  • Newborns with abnormal screening findings should be monitored in accordance with local regulations. Different states or authorities may have various protocols regarding result interpretation and follow-up. Do not restrict dietary phenylalanine or interrupt breastfeeding based on screening results unless instructed by a health official or treatment center. However, immediately refer the patient to a treatment center for confirmatory testing.

  • Low-grade elevations may require repeat screening. Phenylalanine levels can rise for several weeks after birth in children with hyperphenylalaninemia or phenylketonuria (PKU). A low-grade elevation 24-72 hours following birth might signal true PKU, not merely hyperphenylalaninemia.

Measure plasma phenylalanine and tyrosine levels as soon as possible after an abnormal screening result. An elevated phenylalanine level with low or normal tyrosine level is expected. Hepatic insufficiency and tyrosinemia can feature elevated phenylalanine levels, although typically in the context of elevated tyrosine.

Obtain blood and urine biopterins assays through a qualified laboratory to exclude a tetrahydrobiopterin defect.



Diagnostic procedures, such as phenylalanine or biopterin loading tests, are rarely indicated.