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Phenylalanine

Measures phenylalanine, an amino acid from protein foods, and is mostly used to follow phenylketonuria (PKU), which is usually found at birth.

AI-verified against cited sources. Awaiting expert reviewAlso called PheLOINC 14875-9

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What it is

Phenylalanine is an essential amino acid, which means your body cannot make it and must get it from protein in food. An enzyme in the liver normally changes it into another amino acid, tyrosine. The level in your blood reflects how much protein you recently ate and how well that enzyme is working.

Why it matters

In PKU, the enzyme works poorly and phenylalanine builds up in the blood. Left untreated, this can harm the brain, so newborns are screened and treated early with a low-phenylalanine diet and sometimes medicines. After diagnosis, regular phenylalanine tests are the main way to check how well treatment is working, and good control matters before and during pregnancy.

If it’s low

A low level most often means the diet has been low in protein, or that a restricted diet has gone too far. Because your body cannot make phenylalanine, very low levels can cause problems of their own. For people treated for PKU, care teams aim for a target window, not for the lowest possible number.

Common causes

  • Very low protein intake or poor nutrition
  • An overly strict phenylalanine-restricted diet in a person being treated for PKU
  • Illness that reduces appetite and protein intake

What can help

  • Talk with a clinician or dietitian about protein intake
  • If you have PKU, review your diet and formula plan with your metabolic team

If it’s high

A level above the usual range can be a sign that the body is not clearing phenylalanine well, as in PKU or milder forms of high phenylalanine. Other, more common things can raise it too. One result is not enough for a diagnosis, so a clinician will usually look at it with tyrosine and other tests.

Common causes

  • A recent protein-rich meal, or a sample taken without fasting
  • Being born early (prematurity)
  • Liver disease
  • In people already treated for PKU, a higher intake of natural protein than their diet plan allows
  • Reduced activity of the enzyme that breaks down phenylalanine (PKU or a milder form)
  • A problem with the helper molecule BH4, which is behind a small share of cases

What can help

  • Follow up with the clinician or metabolic team that ordered the test
  • If you have PKU, work with a metabolic dietitian and follow your care plan
  • Ask whether a repeat test after a proper fast would help

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on age. Choose an age to see the one that applies.

Children and teens 3 ranges

See Phenylalanine by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • How long since you last ate, and how much protein the last meal held
  • Age: newborns and premature infants have higher usual levels
  • Whether the sample was plasma or a dried blood spot
  • Prematurity and liver disease
  • Pregnancy in a person with PKU, when tighter targets apply
  • A recent protein-rich meal can raise phenylalanine; Mayo Clinic Laboratories requires an 8-hour fast, and infants should be sampled before the next feeding.
  • Plasma should be separated from the cells and sent frozen; ARUP rejects hemolyzed samples.
  • Prematurity, liver disease and a high natural protein intake can raise levels, independent of the enzyme problem behind PKU.

Phenylalanine in the blood follows recent protein intake, so it can change within days of a diet change.

When to talk to a clinician

If you or your child has PKU, contact your metabolic clinic when levels run above the target your team gave you. If a newborn screening result pointed to high phenylalanine, follow the follow-up steps you were given right away. If you have PKU and are pregnant or planning pregnancy, tell your clinician early, because guidelines aim for 120 to 360 µmol/L.

Questions you could ask

  • How often should this be rechecked in my situation?
  • Could a recent meal or something else explain this level?
  • Should we repeat the test after fasting, and would other tests such as tyrosine help?
  • What target range should my care team use for me?
  • Am I getting enough protein in my diet?
  • If I follow a special diet, is my plan too strict?

Questions people ask

How do I convert µmol/L to mg/dL?

Multiply µmol/L by 0.0165 to get mg/dL, so 360 µmol/L is about 6 mg/dL. To go the other way, multiply mg/dL by 60.5.

Why do I need to fast for this test?

Protein from a recent meal raises phenylalanine, which can hide your usual level. Mayo Clinic Laboratories asks for an 8-hour fast, and for infants a sample taken before the next feeding.

Why do the usual ranges differ between labs and ages?

Newborns and young children normally run higher than adults, and each lab builds its own range. For example, Mayo lists 35 to 85 µmol/L for adults and ARUP lists 30 to 82 µmol/L. Compare your result with the range printed on your own report.

Show the science and sources

Units

  • 1 µmol/L = 1 µmol/L
  • 1 µmol/L = 0.01652 mg/dL

Open the Phenylalanine unit converter

Is a change real?

Phenylalanine naturally varies about 9.5% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 29% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).

For clinicians and researchers

Phenylalanine hydroxylase (PAH), with the cofactor tetrahydrobiopterin (BH4), converts phenylalanine to tyrosine; about 2% of infants with hyperphenylalaninemia have BH4 defects (Mayo). Mayo Clinic Laboratories reports phenylalanine and tyrosine together on fasting plasma by LC-MS/MS and notes a phenylalanine:tyrosine ratio above 3 is considered abnormal; it states the test alone cannot establish a diagnosis of hyperphenylalaninemia. Intervals are age-dependent: Mayo adult 35 to 85 µmol/L (nmol/mL), ARUP 30 to 82 µmol/L from age 1 year. Mayo gives mg/dL x 60.5 = µmol/L (MW 165.19). The 2014 ACMG guideline sets lifelong treatment with a blood goal of 120 to 360 µmol/L. The 2017 European guideline defines hyperphenylalaninemia as any blood phenylalanine above 120 µmol/L, treats untreated levels above 600 µmol/L, aims for 120 to 600 µmol/L after age 12, and reports that maternal levels of 120 to 360 µmol/L in early pregnancy were not linked to congenital heart disease in offspring. Its differential for high phenylalanine includes high natural protein intake, prematurity, BH4 defects and liver disease, so PAH deficiency is only one cause.

LOINC 14875-9 (primary), 2765-6. Specimen: plasma.

Sources

  1. Phenylalanine and Tyrosine, Plasma (test PKU), Mayo Clinic Laboratories
  2. Phenylalanine and Tyrosine (test 0080336), ARUP Laboratories
  3. Phenylalanine hydroxylase deficiency: diagnosis and management guideline, Genetics in Medicine (American College of Medical Genetics and Genomics) (2014)
  4. The complete European guidelines on phenylketonuria: diagnosis and treatment, Orphanet Journal of Rare Diseases (2017)
  5. L-Phenylalanine (compound summary, molecular weight), PubChem, National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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