Phenylketonuria screening test
What is this test?
This test measures the amount of an amino acid called phenylalanine in blood. It is used to screen newborns for phenylketonuria (PKU), a genetic disorder wherein the enzyme needed to break down this amino acid is absent or lacking. This causes phenylalanine to build up in the body, which may result in mental retardation and other serious health problems.
What are other names for this test?
- Hyperphenylalaninaemia screen
- Hyperphenylalaninemia screen
- Phenylketonuria screening
- Phenylketonuria test
Why do I need this test?
Laboratory tests may be done for many reasons. Tests are performed for routine health screenings or if a disease or toxicity is suspected. Lab tests may be used to determine if a medical condition is improving or worsening. Lab tests may also be used to measure the success or failure of a medication or treatment plan. Lab tests may be ordered for professional or legal reasons. The following is a possible reason why this test may be done:
When and how often should I have this test?
When and how often laboratory tests are done may depend on many factors. The timing of laboratory tests may rely on the results or completion of other tests, procedures, or treatments. Lab tests may be performed immediately in an emergency, or tests may be delayed as a condition is treated or monitored. A test may be suggested or become necessary when certain signs or symptoms appear.
Due to changes in the way your body naturally functions through the course of a day, lab tests may need to be performed at a certain time of day. If you have prepared for a test by changing your food or fluid intake, lab tests may be timed in accordance with those changes. Timing of tests may be based on increased and decreased levels of medications, drugs or other substances in the body.
The age or gender of the person being tested may affect when and how often a lab test is required. Chronic or progressive conditions may need ongoing monitoring through the use of lab tests. Conditions that worsen and improve may also need frequent monitoring. Certain tests may be repeated to obtain a series of results, or tests may need to be repeated to confirm or disprove results. Timing and frequency of lab tests may vary if they are performed for professional or legal reasons.
The recommended time for this test is between 24 hours and 7 days of life. If blood is collected before 24 hours of life, the test should be repeated when your baby is 2 weeks old.
How should I get ready for the test?
Capillary blood or venous blood:
Before having blood collected, tell the person drawing your blood if you are allergic to latex. Tell the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. Also tell the healthcare worker if you have felt nauseated, lightheaded, or have fainted while having blood drawn in the past.
How is the test done?
A sample of capillary blood or venous blood may be collected for this test.
Common sites to collect a capillary blood sample are the fingertip and earlobe. Infants often have a capillary blood sample taken from the heel of the foot. Once the site is selected, the healthcare worker may heat the area with a warm compress to ensure adequate blood flow. The area will be cleaned with antiseptic. A small needle is used to make a cut in the skin surface, and the area may be squeezed gently to produce blood. The blood is collected in small collection device.
When a blood sample from a vein is needed, a vein in your arm is usually selected. A tourniquet (large rubber strap) may be secured above the vein. The skin over the vein will be cleaned, and a needle will be inserted. You will be asked to hold very still while your blood is collected. Blood will be collected into one or more tubes, and the tourniquet will be removed. When enough blood has been collected, the healthcare worker will take the needle out.
How will the test feel?
The amount of discomfort you feel will depend on many factors, including your sensitivity to pain. Communicate how you are feeling with the person doing the test. Inform the person doing the test if you feel that you cannot continue with the test.
Capillary blood or venous blood:
During a blood draw, you may feel mild discomfort at the location where the blood sample is being collected.
What should I do after the test?
After capillary blood collection is complete, cotton will be placed over the site and held firmly until the bleeding has stopped. A bandage or cotton may be secured over the site.
After a blood sample is collected from your vein, a bandage, cotton ball, or gauze may be placed on the area where the needle was inserted. You may be asked to apply pressure to the area. Avoid strenuous exercise immediately after your blood draw. Contact your healthcare worker if you feel pain or see redness, swelling, or discharge from the puncture site.
What are the risks?
Blood: During a blood draw, a hematoma (blood-filled bump under the skin) or slight bleeding from the puncture site may occur. After a blood draw, a bruise or infection may occur at the puncture site. The person doing this test may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of this test.
What are normal results for this test?
Laboratory test results may vary depending on your age, gender, health history, the method used for the test, and many other factors. If your results are different from the results suggested below, this may not mean that you have a disease. Contact your healthcare worker if you have any questions. The following is considered to be a normal result for this test:
- Newborns: ≤2 mg/dL (≤121 micromol/L) 
What might affect my test results?
Drug Therapy Use:
Some medications may affect the results of the test. These medications include:
- Results increased in:
- Premature infants with immature livers 
- Results decreased in:
- Insufficient protein intake 
What follow up should I do after this test?
Ask your healthcare worker how you will be informed of the test results. You may be asked to call for results, schedule an appointment to discuss results, or notified of results by mail. Follow up care varies depending on many factors related to your test. Sometimes there is no follow up after you have been notified of test results. At other times follow up may be suggested or necessary. Some examples of follow up care include changes to medication or treatment plans, referral to a specialist, more or less frequent monitoring, and additional tests or procedures. Talk with your healthcare worker about any concerns or questions you have regarding follow up care or instructions.
If this test is positive, another test should be done on your baby to confirm the result.
Where can I get more information?
- Henry JB: Clinical Diagnosis and Management by Laboratory Methods, 20th ed. Saunders, 2001.
- Levy HL & Albers S: Genetics screening of newborns. Annu Rev Genomics Hum Genet 2000; 1:139-177.
- Matalon R & Michals K: Phenylketonuria: screening, treatment and maternal PKU. Clin Biochem 1991; 24(4):337-342.
- American Academy of Pediatrics- Committee on Genetics: Newborn screening fact sheets. Pediatrics 1996; 98(3 pt 1):473-501.
- Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.