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Introduction ⬇

Routine blood and urine tests are done on newborns to detect PKU, an autosomal-recessive inherited disease that can lead to intellectual disability and brain damage if untreated. This disease is characterized by a lack of the enzyme that converts phenylalanine, an amino acid, to tyrosine, which is necessary for normal metabolic function. Because dietary phenylalanine is not converted to tyrosine, phenylalanine, phenylpyruvic acid, and other metabolites accumulate in blood and urine. Tyrosine and the derivative catecholamines are deficient, which results in intellectual disability. Both sexes are affected equally, with most cases occurring in persons of northern European ancestry.

This test is used for newborns to detect the metabolic disorder hyperphenylalaninemia. If untreated, this disorder can lead to intellectual disability. Dietary restrictions of phenylalanine have shown good results.

Normal Findings ⬆ ⬇

Blood: <2 mg/dL (2–5 days after birth) or <121 μmol/L

Urine: negative dipstick (detects phenylalanine in range of 5–10 mg/dL or 302–605 μmol/L)

24-Hour urine:

Procedure ⬆ ⬇

The established standard is that all newborn infants should be tested for PKU and congenital hypothyroidism before discharge.

  1. The blood test must be performed at least 3 days after birth or after the child has ingested protein (milk) for at least 24–48 hours.

  2. Urine testing is usually done at the 4- or 6-week checkup if a blood test was not done.

  3. PKU studies should be done on all infants who weigh ≥5 pounds (≥2.3 kg) before they leave the hospital.

  4. Sick or premature infants should be tested within 7 days after birth regardless of protein intake, weight, or antibiotic therapy.

Collecting the Blood Sample

  1. Cleanse the skin with an antiseptic and pierce the infant’s heel with a sterile disposable lancet.

  2. Support the infant, if bleeding is slow, so that the blood flows by means of gravity while spotting the blood with filter paper.

  3. Fill the circles on the filter paper completely. This can best be done by placing one side of the filter paper against the infant’s heel and watching for the blood to appear on the other side of the paper until it completely fills the circle.

  4. Do not touch blood circles until they are completely dry. Keep in cool, dry area.

  5. Transport samples to testing site within 12–24 hours.

  6. Confirm all positive filter paper tests with a quantitative blood or urine test.

Collecting the Urine Sample in Nursery or at Home

  1. Dip the reagent strip into a fresh sample of urine or press it against a wet diaper (phenylalanines and phenylpyruvic acid may not appear in urine until the infant is 2–3 weeks of age).

  2. After exactly 30 seconds, compare the strip with a color chart according to manufacturer’s directions.

  3. Salicylates and phenothiazine may cause abnormal color reactions.

  4. All positive tests must be confirmed with a quantitative chemical test.

Collecting the 24-Hour Urine Sample

  1. Properly label a 24-hour clean-catch urine container with the patient’s name, date and time of collection, and test(s) ordered.

  2. Provide refrigeration or icing. Check with your laboratory regarding the need for preservatives (e.g., 10 mL of 6 mol/L HCl).

  3. Follow general instructions for 24-hour urine collection (see Long-Term, Timed Urine Specimen [2-Hour, 24-Hour]).

  4. Record exact starting and ending times on the specimen container and in the patient’s healthcare record.

  5. Send the specimen to the laboratory.

Clinical Implications ⬆ ⬇

Increased phenylalanine is found in:

  1. Hyperphenylalaninemia. In a positive test for PKU, the blood phenylalanine is >15 mg/dL or >907 μmol/L. Blood tyrosine is <5 mg/dL or <276 μmol/L; it is never increased in PKU.

  2. Obesity

  3. In low-birthweight or premature infants, transient hyperphenylalaninemia, along with transient hypertyrosinemia, may occur.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain purpose of test, procedure for specimen collection, and interfering factors to the caregiver.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results with the caregiver; report and record findings. Modify the nursing care plan as needed. Monitor appropriately.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Premature infants, those weighing <5 pounds (<2.3 kg), may have elevated phenylalanine and tyrosine levels without having the genetic disease. This is a result of delayed development of appropriate enzyme activity in the liver (liver immaturity).

  2. Antibiotics interfere with the blood assay.

  3. Cord blood cannot be used for analysis.

  4. Two days of protein feeding must be done before blood is taken.