Skip to main content Skip to navigation

Serum Phenylalanine Screening – Indications, Procedures, Results

Last updated: February 8, 2026 Medically reviewed by: RxHarun Orthopedic Specialist Doctor Reading time: 8 min read

Serum phenylalanine screening is a blood test to look for signs of the disease phenylketonuria (PKU). The test detects abnormally high levels of an amino acid called phenylalanine. A PKU screening test is a blood test given to newborns 24–72 hours after birth. PKU stands for phenylketonuria, a rare disorder that prevents the body from properly breaking down a substance called phenylalanine (Phe). A dangerous buildup of phenylalanine can develop when a person with PKU eats protein-rich foods, such as milk, cheese, nuts, or meat, and even grains such as bread and pasta, or eats aspartame, an artificial sweetener. This buildup of phenylalanine results in damage to nerve cells in the brain.

How the Test is Performed

The test is usually done as part of routine screening tests before a newborn leaves the hospital. If the child is not born in a hospital, the test should be done in the first 48 to 72 hours of life.

An area of the infant’s skin, usually the heel, is cleaned with a germ killer and punctured with a sharp needle or a lancet. Three drops of blood are placed in 3 separate test circles on a piece of paper. Cotton or a bandage may be applied to the puncture site if it is still bleeding after the blood drops are taken.

The test paper is taken to the laboratory, where it is mixed with a type of bacteria that needs phenylalanine to grow. Another substance that blocks phenylalanine from reacting with anything else is added.

See also: Newborn screening tests

How to Prepare for the Test

For help preparing your baby for the test, see the infant test or procedure preparation (birth to 1 year).

How the Test will Feel

When the needle is inserted to draw blood, some infants feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing. Infants are given a small amount of sugar water, which has been shown to reduce the painful sensation associated with skin punctures.

Why the Test is Performed

This test is done to screen infants for PKU, a fairly rare condition that occurs when the body lacks a substance needed to break down the amino acid phenylalanine.

If PKU is not detected early, increasing phenylalanine levels in the baby will cause intellectual disability. When discovered early, changes in the diet can help prevent the severe side effects of PKU.

Normal Results

A normal test result means that phenylalanine levels are normal and the child does not have PKU.

Normal value ranges may vary slightly among different laboratories. Talk to your health care provider about the meaning of your baby’s test results.

What Abnormal Results Mean

If the screening test results are abnormal, PKU is a possibility. Further testing will be done if the phenylalanine levels in your baby’s blood are too high.

Risks

The risks of having blood drawn are slight, but include:

  • Excessive bleeding
  • Fainting or feeling lightheaded
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)
  • Multiple punctures to locate veins

FAQ

Who is screened for high levels of phenylalanine in the blood?

  • What is a PKU screening test? A PKU screening test is a blood test given to newborns 24–72 hours after birth. PKU stands for phenylketonuria, a rare disorder that prevents the body from properly breaking down a substance called phenylalanine (Phe).

What does elevated phenylalanine mean?

  • A dangerous buildup of phenylalanine can develop when a person with PKU eats protein-rich foods, such as milk, cheese, nuts or meat, and even grains such as bread and pasta, or eats aspartame, an artificial sweetener. This buildup of phenylalanine results in damage to nerve cells in the brain.

What is a normal phenylalanine level?

  • The test screens for blood levels of phenylalanine. Normal levels of phenylalanine in the blood are less than 2 milligrams per deciliter (mg/dL). More than 4 mg/dL of phenylalanine in the blood is considered high and may mean your child has PKU.

Which screening test can detect phenylketonuria?

  • Newborn blood testing identifies almost all cases of phenylketonuria.

What is serum phenylalanine?

  • Serum phenylalanine screening is a blood test to look for signs of the disease phenylketonuria (PKU). The test detects abnormally high levels of an amino acid called phenylalanine.

What does a positive Guthrie test mean?

  • Phenylketonuria (PKU) is a congenital birth abnormality in which toxic levels of the amino acid phenylalanine build up in the blood, a process that affects the brains of untreated infants. Guthrie’s test detects phenylalanine in the blood of newborns, enabling early diagnosis of PKU.

What are the dangers of phenylalanine?

  • Phenylalanine can cause intellectual disabilities, brain damage, seizures, and other problems in people with PKU. Phenylalanine occurs naturally in many protein-rich foods, such as milk, eggs, and meat.

How does phenylalanine affect the brain?

  • The high plasma phenylalanine concentrations increase phenylalanine entry into the brain and restrict the entry of other large neutral amino acids. In the literature, emphasis has been on high brain phenylalanine as the pathological substrate that causes mental retardation.

What happens if PKU is left untreated?

  • Children with PKU who are not treated may develop symptoms including behavioral problems, seizures, and severe intellectual and developmental disabilities. Adults with PKU who do not follow a special diet may develop unstable moods2 and take longer to process information.

At what age does PKU become evident?

  • Babies who have PKU seem normal for the first few months of life. But without treatment, they begin to show signs and symptoms of the illness at about 6 months of age.

What does a positive PKU test indicate?

  • Positive phenylketonuria (PKU) test: The test looks for phenylalanine levels in the blood, which should be less than 2 mg/dL. A blood phenylalanine level of more than 4 mg/dL is considered excessive and could indicate that the child has PKU.

Is Hyperphenylalaninemia the same as PKU?

  • Benign hyperphenylalaninemia (H-PHE) is a form of phenylketonuria (PKU). Different forms of PKU have varying severity of signs. Because H-PHE is a less severe type of PKU, babies with H-PHE typically do not show any signs.

How much phenylalanine is too much?

  • Doses higher than 5,000 milligrams a day can cause nerve damage. Risks. People with certain conditions should avoid using this supplement, including those with schizophrenia (tardive dyskinesia, a movement disorder, may develop.)

What are the benefits of phenylalanine?

  • Phenylalanine is involved in the production of the neurotransmitters norepinephrine and dopamine, which are essential for the proper functioning of the brain and nervous system. Phenylalanine also helps with the production of the pigment melanin, which gives color to the skin, hair, and eyes.

Does phenylalanine raise blood pressure?

  • Phenylalanine can increase a chemical in the body called tyramine. Large amounts of tyramine can cause high blood pressure.

What is the life expectancy of a person with PKU?

  • Diagnosing PKU
  • Treatment includes a special diet and regular blood tests. With early diagnosis and the correct treatment, most children with PKU are able to live healthy lives. About 1 in 10,000 babies born in the UK has PKU.

What foods should be avoided with PKU?

  • People with PKU must avoid foods that are high in protein like meat, fish, poultry, dairy, soy, legumes (dried beans) or nuts. Some fruits and vegetables are higher in protein than others. PHE is in almost everything except sugar, salt, oil, and water. Thankfully, many fruits and vegetables are naturally low in PHE.

What’s the difference between L-phenylalanine and DL-phenylalanine?

  • The key difference between L-phenylalanine and DL-phenylalanine is that L-phenylalanine is the L isomer of phenylalanine whereas DL-phenylalanine is the racemic mixture of D and L isomers phenylalanine. Moreover, L-phenylalanine is common and stable in nature while DL-phenylalanine can be found in trace amounts.

What is the diet for PKU?

  • 1 A PKU diet centers around foods that are low in phenylalanine to keep this build-up to a minimum. Avoiding protein-rich foods is one of the basics of the PKU diet, as these choices are a major source of phenylalanine. Of course, your body still needs protein, so specially formulated medical foods help fill that gap.

What PKU affects the brain.

  • When neurotransmitters are not made in the right amounts, the brain cannot function properly. High blood Phe levels can cause disruptions in neurotransmitters like serotonin and dopamine, which are important for mood, learning, memory, and motivation.

What happens if someone with PKU eats protein?

  • A dangerous buildup of phenylalanine can develop when a person with PKU eats protein-rich foods, such as milk, cheese, nuts, or meat, and even grains such as bread and pasta, or eats aspartame, an artificial sweetener. This buildup of phenylalanine results in damage to nerve cells in the brain.

What are the dangers of phenylalanine?

  • Phenylalanine can cause intellectual disabilities, brain damage, seizures, and other problems in people with PKU. Phenylalanine occurs naturally in many protein-rich foods, such as milk, eggs, and meat.

Why phenylalanine is toxic?

  • This enzyme converts the amino acid phenylalanine to other important compounds in the body. If gene mutations reduce the activity of phenylalanine hydroxylase, phenylalanine from the diet is not processed effectively. As a result, this amino acid can build up to toxic levels in the blood and other tissues.

What happens if PKU is left untreated?

  • Children with PKU who are not treated may develop symptoms including behavioral problems, seizures, and severe intellectual and developmental disabilities. Adults with PKU who do not follow a special diet may develop unstable moods2 and take longer to process information.

Frequently Asked Questions

Is this article a replacement for a doctor?

No. It is educational content only. Patients should consult a qualified clinician for diagnosis and treatment.

When should I seek urgent care?

Seek urgent care for severe symptoms, rapidly worsening condition, breathing difficulty, severe pain, neurological changes, or any emergency warning sign.

References

Add references, clinical guidelines, textbooks, journal articles, or trusted medical sources here. You can edit this area later with a custom field named _rx_references.

Related Articles

Hemoglobin A1c Test

November 6, 202521 min read0 views

The hemoglobin A1c test—also known as glycated hemoglobin, glycosylated hemoglobin, HbA1c, or simply A1c—is used to measure an individual’s glucose control levels. The test shows average…

Read more

Orbital Decompression Surgery

August 18, 202518 min read0 views

Orbital decompression is an operation to make more room inside the eye socket. The eye socket is called the “orbit.” It is a cone-shaped…

Read more

Olfactory Nerve Disorders

May 8, 20248 min read0 views

Olfactory nerve disorders affect our sense of smell. This can impact our ability to enjoy food, detect danger like gas leaks, or even notice…

Read more

Written by Dr. Harun Ar Rashid, MD - Arthritis, Bones, Joints Pain, Trauma, and Internal Medicine Specialist

Dr. Md. Harun Ar Rashid, MPH, MD, PhD, is a highly respected medical specialist celebrated for his exceptional clinical expertise and unwavering commitment to patient care. With advanced qualifications including MPH, MD, and PhD, he integrates cutting-edge research with a compassionate approach to medicine, ensuring that every patient receives personalized and effective treatment. His extensive training and hands-on experience enable him to diagnose complex conditions accurately and develop innovative treatment strategies tailored to individual needs. In addition to his clinical practice, Dr. Harun Ar Rashid is dedicated to medical education and research, writing and inventory creative thinking, innovative idea, critical care managementing make in his community to outreach, often participating in initiatives that promote health awareness and advance medical knowledge. His career is a testament to the high standards represented by his credentials, and he continues to contribute significantly to his field, driving improvements in both patient outcomes and healthcare practices. Born and educated in Bangladesh, Dr. Rashid earned his BPT from the University of Dhaka before pursuing postgraduate training internationally. He completed his MD in Internal Medicine at King’s College London, where he developed a special interest in inflammatory arthritis and metabolic bone disease. He then undertook a PhD in Orthopedic Science at the University of Oxford, conducting pioneering research on cytokine signaling pathways in rheumatoid arthritis. Following his doctoral studies, Dr. Rashid returned to clinical work with a fellowship in interventional pain management at the Rx University School of Medicine, refining his skills in image-guided joint injections and minimally invasive pain-relief techniques.