Newborn Metabolic Screen
Edward F. Bell, MD
Peer Review Status: Internally Peer Reviewed
In Iowa, all newborns are screened by the Iowa Birth Defects Institute for hypothyroidism, phenylketonuria, galactosemia, maple syrup urine disease, hemoglobinopathies, and adrenal hyperplasia. In Iowa and many other states, extended screening for many additional metabolic disorders is now done routinely using tandem mass spectrometry. Practical questions regarding newborn screening are addressed in an article in the Iowa Perinatal Letter in April 2008.
Criteria for screening include:
- A disorder that is sufficiently common to justify screening.
- A relatively simple, accurate and inexpensive screening test should be available, that has a high sensitivity with a high negative predictive value.
- Treatment should be available for the disorder, and there should be a demonstrable benefit to starting treatment before clinical symptoms appear and the diagnosis is made on clinical grounds.
- The test should be possible from the spot of blood obtained on the NNS card.
Screening is mandated by Iowa State Law [Code of Iowa, Chapter 4:641 (136A)]. The ultimate responsibility for screening newborns rests with the attending physician. Should a parent refuse the test, they must sign a waiver form which is available in the Nursery. This waiver shall become a part of the medical record and a copy sent to the Birth Defects Institute of the Department of Public Health.
The information portion of the screening form will be filled out by the ward clerk. The circles on the filter paper should be filled almost to the black lines. At present, one additional drop of blood should be added. Each circle should be filled with a single large drop of blood obtained by heelstick or from an indwelling catheter. Multiple small drops will result in layering, which will give inaccurate results. Blood obtained through an umbilical catheter is not acceptable for testing if medications or parenteral nutrition solutions have recently been administered.
Screening should be done prior to discharge, ideally no sooner than 48 hours after birth (to allow phenylalanine levels to rise) and before 5 days.
Repeat screen immediately if:
- specimen rejected because of poor quality, insufficient blood, slip is incomplete or give incorrect demographic data.
- a presumptive positive on previous screen.
Repeat by 14 days if first NNS:
- done at < 48 hr. age - test for PKU and MSUD may be false-negative as blood levels for the amino acids may be normal at birth.
- Infant on antibiotics - must wait 24 hours after last dose to repeat NNS. If the infant is to go home before twenty-four hours, the repeat test should be performed at the time of discharge, and an additional repeat screening should be performed by the infant’s physician by 14 days age.
Multiple births may affect test results. Be sure to indicate - especially if twin-twin transfusion involved.
- can potentially affect all tests.
- re-test 6 weeks after last transfusion to be certain of test results.
If positive results
- the patient’s UIHC physician will be notified by the University Hygienic Laboratory.
- confirmatory diagnostic tests and treatment should follow.
A very complicated group of diseases that involve defects in the kind or the amount of hemoglobin in red blood cells. Early detection identifies families at risk for having future affected children and allows early treatment of affected infants. The major disorders of clinical significance in the US are sickle cell disease and hemoglobin C disease.
Iowa detects abnormalities in the a and b chains including:
- Hemoglobin-S, C, E, O, G, and D.
- Also detects Bart’s hemoglobin in a-thalassemia.
Because of the high levels of hemoglobin F at birth, it may be difficult to accurately quantitate the hemoglobins. However, identification of hemoglobin S, C etc., defines a group of infants who should have a quantitative electrophoresis repeated at 3 - 6 mo. age. The test is done by isoelectric focusing and confirmed by high pressure liquid chromatography (HPLC). The test is affected by red blood cell transfusions and should not be performed within 6 weeks following a transfusion.
1 in 10-15,000 births. Autosomal recessive. Deficiency of liver enzyme phenylalanine hydroxylase (PH) that metabolizes phenylalanine to tyrosine. Gene for PH is on chromosome 12q, with > 20 different mutations identified.
Currently, all 50 states screen newborns for this disease. Phenylalanine levels are normal in the cord blood of neonates and only rise after milk feedings have been initiated, hence screens obtained prior to 48 hours may give false negative results. In cases of infants discharged before 24 hours, most states recommend that a sample be obtained at discharge and subsequently repeated. In the case of sick newborns, who are not being fed, the initial newborn screen should not be withheld indefinitely but should be submitted at the recommended time, because screening for some other disorders, such as hypothyroidism is not affected by the feeding history. After feedings are started, a repeat screen should be submitted for PKU. Since a bacterial inhibition assay is used for screening, antibiotics given to the infant can interfere as well, hence screens should be repeated 24 - 48 hr. after stopping antibiotics.
Severe deficiency - classical form; Milder deficiency - variant forms
Leads to accumulation of phenylalanine and its metabolic byproducts in the blood and urine of affected individual. The urine of affected individuals has a peculiar musty odor. Brain myelination is abnormal. PKU should be considered in an infant who loses developmental milestones in the first 6 to 12 months of life.
Dietary restriction of phenylalanine is highly successful in preventing mental retardation, but the diet is unpalatable. A special formula (Lofenalac) low in phenylalanine is utilized in order to provide other necessary nutrients. However, Lofenalac alone is not an adequate diet for any infant, including infants with PKU, and caution should be exercised in placing children on low phenylalanine diets unless the diagnosis has been clearly established, and only after consultation with a center experienced in the management of PKU. Dietary restriction was once maintained for 5 to 6 years of age and then discontinued. However, some patients exhibited neurologic deterioration and loss of IQ points after discontinuation of the diet. In addition, this leads to potential adverse effects of maternal metabolic derangementis on fetal growth and development. Treatment is now maintained indefinitely in most cases.
In the past, patients with PKU were severely retarded and did not reproduce. This changed completely with the initiation of newborn screening and early dietary management. It is especially important that affected women resume a strict dietary avoidance of phenylalanine prior to, during, and after pregnancy (esp. for those wishing to breast feed their infants), so that the fetus/infant does not see abnormally high levels of phenylalanine. For anyone who has been on a normal diet, the phenylalanine-restricted diet is a onerous diet, and is a very difficult goal to achieve. The maternal metabolic environment in this condition has extremely harmful effects on fetal development. Over 90% of infants of these mothers (most of whom do not themselves have PKU) are affected. These infants exhibit mental retardation (>90%), microcephaly (72%), growth retardation (40%), congenital heart disease (12%). The risk of these abnormalities is correlated with the mother’s blood phenylalanine level.
1 in 50-75,000 births, or 2 infants in Iowa per year. Autosomal recessive.
Deficiency of either
- galactose-1-phosphate uridyl transferase (GPUT) or
- galactokinase (GK) or
- uridine diphosphate galactose-4-epimerase
Galactose ----GK^ Gal-1-P + UDPG ----GPUT^ glucose-1-P + UDP-galactose
Classical galactosemia generally presents in the newborn period with failure to thrive, jaundice, hepatomegaly and renal failure. If unrecognized, death may result by 4 to 10 days of age, or a chronic course of cirrhosis, cataracts (w/ galactokinase deficiency), brain damage, seizures, and mental retardation may ensue. Liver biopsy shows marked fatty accumulation in hepatocytes and fibrosis that may be quite extensive even in the first weeks of life. Newborns have a greater risk of infection (esp. E-coli sepsis) if treatment is delayed.
The screening test is done by Beutler immunofluorescence and is affected by transfusions, as the RBCs contain the enzyme necessary for glycolysis. Dried blood disks are mixed with Gal-1-P and UDPG, and if the reaction is completed by GPUT from the infant’s blood, NADPH is made and detected by fluorescence. Note, this test does not detect galactokinase or epimerase deficiency, but will detect transferase deficiency regardless of prior dietary intake of galactose.
A positive test should be treated as a medical emergency. Immediately institute a strict diet low in lactose, galactose and milk solids (e.g. soy formula). Confirm diagnosis quickly by direct enzyme and galactose-1-phosphate measurement in red cells, and test for presence of urine reducing substances. It is very difficult to maintain strict dietary restriction because of the ubiquitous use of lactose as a food additive. Dietary treatment should be continued throughout the persons life.
1 in 5000 births, or 9 infants/yr. in Iowa;
In North America, Female : male is 2 : 1.
A variety of disturbances of morphogenesis involving the hypothalamic-pituitary axis and the thyroid gland may result in congenital deficiency of thyroxine. If undetected, the deficiency results in severe mental and growth retardation. Initial signs appearing over the first few weeks of life include lethargy, hypothermia, hypoactivity, hypotonia, large anterior and posterior fontanels, poor feeding, respiratory distress (myxedema of the airway), perioral cyanosis, pallor, poor or hoarse cry, mottled skin, constipation, and prolonged physiologic jaundice. The classic features of cretinism usually appear after 6 weeks of life, including the typical facies (depressed nasal bridge, narrow forehead, puffy eyelids, thick dry cold skin, coarse hair), abdominal distention, umbilical hernia, and large cranial fontanels.
Test done by radioimmunoassay - T4 assayed as initial screen; each day bottom 5 -10% are assayed for TSH. False positives occur frequently in LBW and premature infants.
Indications for re-testing: (normal level of T4 7 -10 mcg/dl in infants)
- T4 5-7 mcg/dl with normal TSH (<20) - repeat q month until T4 > 7
- T4 3-5 mcg/dl with normal TSH - repeat q 14 d until > 5, then every month until > 7
- T4 < 3 mcg/dl - obtain serum T4, free T4 and TSH
The prognosis and outcome are greatly improved by early diagnosis and treatment with thyroid hormone replacement begun prior to 3 months of age. Confirmatory testing for primary hypothyroidism should be done prior to 3 weeks of age.
1 in 150,000 - 206,000 births, or 1 infant in Iowa every 1 to 4 years.
Branched chain ketoacidemia due to congenital deficiency of branched-chain ketoacid dehydrogenase (BCKD) that initiates degradation of branched-chain amino acids (BCAA) leucine, isoleucine, and valine. Urine has a sweet syrupy odor. If untreated the severe neonatal form leads to poor feeding, vomiting, tachypnea or irregular respirations, ketoacidosis, hypoglycemia and progressive neurologic dysfunction - rigidity alternating with periods of lethargy, or seizures, and often death. If the patient survives the initial episode, the disorder leads to severe mental and motor retardation, growth failure, hypertonia and seizures. The disorder is fatal if not appropriately treated. Death can occur by 4 to 7 days age due to acidosis and hypoglycemia.
Test done by bacterial inhibition assay; test can be affected by antibiotic use. Confirm diagnosis by specific blood and urinary testing looking for the characteristic large amounts of these three amino acids in the blood, and the urinary a ketoacids and organic acid patterns.
Given the rapid onset of this disease, immediately institute a special diet with reduced BCAA intake (amino-acid mix free of leucine, isoleucine, and valine) with added oil and dextromaltose. Since a minority of patients have thiamine-responsive defects some cases, supplementation with pharmacological doses of thiamine, a co-factor for the first component of the enzyme BCKD, is recommended. In practice, these patients require complex treatment and should immediately be referred to a center experienced in the management of patients with MSUD.
1 in 12,000-14,000 births
Deficiency or abnormal form of one of five enzymes involved in adrenal steroid synthesis - leads to inability to synthesize the stress hormone cortisol, causing a secondary increase in ACTH. ACTH in turn stimulates adrenocortical hyperplasia and increases adrenocortical steroidogenic activity in an attempt to normalize cortisol production. The 21- and 11b-hydroxylase deficiencies, and to a lesser extent deficiency of 3b-hydroxysteroid dehydrogenase) result in excess formation of precursors steroids, which leads to excess androgen production that induces masculinization of affected female fetuses in utero. Affected females (pseudohermaphroditism) should be identifiable at birth. Affected males with 21- or 11b-hydroxylase deficiency will develop penile enlargement or other virilization postnatally if untreated. Affected males with 3b-hydrosteroid dehydrogenase deficiency will have ambiguous genitalia because of testosterone deficiency. Except for 17b-hydroxylase, the five enzymes involved in the synthesis of cortisol from cholesterol are also necessary for mineralocorticoid (aldosterone) biosynthesis. 21-hydroxylase deficiency is the most common cause of CAH accounting for 90% of cases. Three forms are recognized. Two forms are seen in neonates - a simple virilizing form (partial deficiency), and a salt losing form (a more complete deficiency) that may present at 1 - 2 weeks age with adrenal crisis, history of poor feeding and vomiting, and have profound hyponatremic dehydration, acidosis, hypoglycemia, and hyperkalemia. There is also a late-onset very mild enzyme deficiency that does not have clinical manifestations in the fetus, neonate, or infant.
Test is done by radioactive immunoassay - quite accurate
False positives seen in premature infants - Re-test them as soon as possible
Provide physiological replacement of end products (cortisol, aldosterone, gonadol sex steroid), i.e. cortisone, hydrocortisone, florinef, sodium supplementation etc.