15: Newborn Screening

This is a population-based system for the identification and early treatment of potentially devastating medical conditions. In the United States, this screening is mandated in every state, but the disorders included in the screening panels vary. A list of the screening tests provided by each state can be found on the National Newborn Screening and Genetics Resource Center website at http://genes-r-us.uthscsa.edu. An expert panel commissioned by the American College of Medical Genetics (ACMG) has recommended 29 conditions on newborn screening panels (Table 15–1). This chapter addresses selected disorders included in most state newborn screen panels, as well as special considerations related to discharge planning and follow-up. The American Academy of Pediatrics (AAP) has published newborn screening FACT sheets that address appropriate testing methods, follow-up, and diagnostic testing for all 29 disorders recommended by the ACMG.

1. An initial specimen. It is typically drawn between 24 and 72 hours of life.

2. Obtain the specimen prior to blood transfusion. It is recommended practice to obtain a specimen prior to blood transfusion if a specimen has not already been sent.

3. Invalid results. Any results judged to be invalid by the performing laboratory or any results that are reported as positive must have a repeat specimen.

   1. Antibiotics. It is important to note that antibiotic administration will increase the chance of a false-positive NBS, which will require a repeat NBS to ensure that the patient does not have the underlying disorder.

   2. Preterm infants. These infants are much more likely to have an abnormal newborn screen due to hepatic immaturity, parenteral nutrition, blood transfusions, and delayed enteral nutrition, which will frequently require these patients to have multiple NBS sent during their NICU course.

1. Amino acid metabolism disorders

   1. Phenylketonuria (PKU)

      1. Screening process. Increased concentrations of phenylalanine in the blood can be detected using fluorometric analysis and tandem mass spectrometry (sometimes referred to as MS/MS).

      2. Follow-up. Children diagnosed with PKU should receive diets with reduced phenylalanine. Subspecialty follow-up should be arranged with a nutritionist as well as with a pediatrician who specializes in metabolic disorders.

2. Organic acid metabolism disorders

   1. Methylmalonic acidemia (MMA)

      1. Screening. Elevated levels of propionylcarnitine are detected by tandem mass spectrometry.

      2. Follow-up. Abnormal screening results should be followed with plasma and/or urine organic acid analysis, which can establish the presence or absence of methylmalonic acid. A high level of methylmalonic acid is diagnostic. Treatment includes the institution of a low-protein diet. Follow-up care should be arranged with a pediatrician who specializes in metabolic disorders, as well as with a nutritionist. MMA has an autosomal recessive inheritance pattern. Genetic counseling should be provided.

3. Fatty acid oxidation disorders

   1. Medium-chain acyl-CoA dehydrogenase deficiency (MCAD)

      1. Screening. Tandem mass spectrometry is the screening tool of choice for MCAD. Levels of octanoylcarnitine are higher during the first 3 days of life, so screening is best performed in the newborn period. Prematurity, immaturity of hepatic function, and total parenteral nutrition can result in abnormal amino acid results, necessitating a repeat specimen.

      2. Follow-up. Diagnostic testing is done though plasma acylcarnitine analysis and urinary organic acid analysis. Molecular analysis to determine the particular gene involved provides prognostic information.

4. Hemoglobinopathies

   1. Sickle cell anemia/disease

      1. Screening. Hemoglobin variants are detected using isoelectric focusing, high-performance liquid chromatography (HPLC), or cellulose acetate electrophoresis. Retesting of abnormal screen samples is usually done by electrophoretic technique, HPLC, immunologic testing, or DNA assays. Blood transfusions that include red cell components can invalidate the screen results, so initial screening should be done prior to transfusion. (Plasma, platelets, and albumin transfusions do not affect screening results.)

      2. Follow-up. The majority of states require follow-up screening on normal screens 90 days after the last transfusion. Confirmatory testing should be done on abnormal screens before 2 months of age. A pediatric hematologist should be involved in follow-up care.

5. Other

   1. Congenital hypothyroidism (CH). See also Chapter 140.

      1. Screening. The primary measurement for most screens is thyrotropin/thyroid stimulating hormone (TSH) and T₄. Optimal timing is between 48 hours and 4 days of age. False-positive elevations in TSH may occur in infants screened at <48 hours of age due to a thyrotropin surge after birth. This surge may be delayed in preterm infants. Likewise, preterm infants with hypothyroidism have delayed elevation of TSH levels, presumably due to immaturity of the hypothalamic-pituitary-thyroid axis. Therefore, it may be prudent to consider a second routine screening on preterm infants between 2 and 6 weeks of age. Preterm infants also typically have lower T₄ levels, resulting in more false-positive results. Screening for hypothyroidism is unaffected by diet or transfusion, except in the case of total exchange transfusion.

      2. Follow-up. Abnormal screening results should be immediately followed up with TSH testing and free T₄. Etiology of hypothyroidism can be determined using thyroid ultrasound, thyroid uptake and scan, and/or thyrotropin-binding inhibitor immunoglobulin (in cases of suspected transient hypothyroidism related to maternal autoimmune thyroid disease). Diagnostic testing to determine etiology is optional but does not alter treatment, and it should never delay treatment. Prompt consultation with a pediatric endocrinologist and early initiation of adequate thyroid replacement therapy is paramount to improving lifelong outcome.

   2. Cystic fibrosis (CF)

      1. Screening. It is estimated that ∽15% of patients with CF have had a neonatal presentation, making CF an ideal disorder to be evaluated through NBS. Determination of sweat chloride concentrations is the hallmark diagnostic test for CF; however, it is impractical for most newborn infants, especially preterm infants of <36 weeks' gestation or a birthweight of <2000 g, and is not recommended. Alternative screening tests include immunoreactive serum trypsinogen (IRT) and fecal elastase-1. More specifically, genotyping can identify the more common mutations involving the CF transmembrane regulator, of which the ΔF508 mutation is the most frequently identified. Neonates diagnosed with meconium ileus (MI) should eventually have a sweat chloride test performed, due to the possibility of a false-negative IRT; 80–90% of patients with MI will have CF.

      2. Follow-up. Neonates who have a positive screen will require DNA testing to evaluate for their particular gene mutation. Additional follow-up should include referral to a pediatric pulmonologist for all infants who have a positive CF screening test or who have equivocal results on their sweat chloride test.

   3. Biotinidase deficiency (BIOT)

      1. Screening. Filter paper spotted with whole blood is assessed for biotinidase activity by a semiquantitative colorimetric method. Up to 20% of patients with symptomatic biotinidase can be missed if screened with tandem mass spectrometry, so this methodology should not be used.

      2. Follow-up. Some states require a repeat specimen between 1 week and 4 months after the last red blood cell transfusion. A serum specimen for quantitative measurement of biotinidase activity should be obtained if the screening results were positive. Children with BIOT should be followed by someone who specializes in metabolic disorders for biotin therapy.

   4. Galactosemia (GALT)

      1. Screening. Three different enzyme deficiencies may result in GALT. Classic galactosemia, which is the most common form, results from the deficiency of galactose 1-phosphate (GALT). Galactokinase (GALK) deficiency and galactose-4-epimerase (GALE) deficiency also result in galactosemia but are very rare. Screening tests vary by state and measure galactose, galactose 1-phosphate plus galactose, and/or GALT enzyme deficiency, which can be done using MS/MS. The GALT enzyme test is performed using red blood cells and is diagnostic only for classic galactosemia. It is not affected by the infant's diet but rather by red blood cell transfusions. Test results of galactose and galactose 1-phosphate are influenced by the infant's diet, so the infant should receive a galactose-containing formula or breast milk prior to screening.

      2. Follow-up. A false-negative result may persist for 3 months after a red blood cell transfusion, so a repeat test is needed 90 days after the last transfusion. All infants with positive screening results should receive a nutrition consultation and be placed on a galactose-restricted diet pending definitive diagnostic testing. Breast-milk feedings are contraindicated for infants with galactosemia, and soy-based formulas are commonly used. Quantitative analysis of galactokinase (GALK) and galactose-4-epimerase (GALE) identifies these less common types of galactosemia.

   5. Congenital adrenal hyperplasia (CAH)

      1. Screening. Measurement of 17-hydroxyprogesterone (17-OHP) is performed through a variety of reagents/immunoassays or tandem mass spectrometry. A high rate of false-positive results occurs in samples taken at <1 day of age. Normal 17-OHP levels are affected by birthweight and gestational age; thus, prematurity and illness can result in false-positive screen results. Preterm infants tend to have higher levels. Levels are not affected by transfusions if they are drawn several hours after the transfusion. However, some states require a repeat specimen with the timing of the repeat screen varying from 72 hours to 4 months after transfusion.

      2. Follow-up. Abnormal screening results should be followed with a serum 17-OHP level. Serum electrolytes and serum 17-OHP levels should be obtained in neonates with a disorder of sex development. An adrenocorticotropic hormone stimulation test is useful for ruling out nonclassic CAH in infants with mild elevations of 17-OHP levels. Infants with CAH should be followed by a pediatric endocrinologist.

   6. Hearing loss (HEAR)

      1. Screening. Universal screening should be done before 1 month of age and/or prior to initial hospital discharge. Although automated auditory brainstem response (ABR) and otoacoustic emission are the primary methods used, ABR is the recommended screening tool for infants with neonatal intensive care unit (NICU) stays >5 days because of the high risk of neural hearing loss.

      2. Follow-up

         1. Audiologic testing and medical follow-up should be performed prior to 3 months of age for all infants who do not pass the initial screen. Rescreening of both ears is recommended.

         2. Infants who have one or more risk factors should be reassessed by an audiologist no later than 24–30 months of age regardless of initial screening results. Risk factors include the following: family history of sensorineural hearing loss, perinatal infections, craniofacial anomalies, hyperbilirubinemia necessitating exchange transfusion, ototoxic medications including chemotherapy, treatment with extracorporeal membrane oxygenation, bacterial meningitis, mechanical ventilation, neurodegenerative disorders, syndromes known to include hearing loss, or trauma.

         3. Infants with confirmed hearing loss should be evaluated by a pediatric otolaryngologist, a pediatric ophthalmologist for assessment of visual acuity, and early intervention services no later than 6 months of age. A genetic consultation should also be considered.