What tests are included in the newborn screen?

Newborn Screening Tests in the United States

Newborn screening in the United States includes tests for medium-chain acyl CoA dehydrogenase (MCAD) deficiency, congenital hypothyroidism (CH), phenylketonuria (PKU), biotinidase deficiency (BIOT), sickle cell anemia (Hb SS), and congenital adrenal hyperplasia (CAH), which are the highest scoring conditions in the American College of Medical Genetics evaluation system. 1

Core Conditions in Newborn Screening

Newborn screening is a state-based public health system that tests almost all (≥97%) of the 4 million babies born in the United States each year using dried blood spots collected shortly after birth 1, 2. The core panel includes:

• Metabolic disorders:

  • Phenylketonuria (PKU) - one of the first conditions screened for since the 1960s 1, 3
  • Medium-chain acyl CoA dehydrogenase (MCAD) deficiency - highest scoring condition in evaluation systems 1
  • Maple syrup urine disease - detected through amino acid analysis 1, 3
  • Homocystinuria - requires persistent abnormal metabolite levels for diagnosis 1, 3
  • Biotinidase deficiency - among top scoring conditions 1
  • Galactosemia - included in early screening programs 4, 3
  • Tyrosinemia - included in expanded screening panels 3

• Endocrine disorders:

  • Congenital hypothyroidism (CH) - one of the highest scoring conditions 1, 3
  • Congenital adrenal hyperplasia (CAH) - screened using tandem mass spectrometry to measure 17α-hydroxyprogesterone levels 5, 3

• Hemoglobinopathies:

  • Sickle cell anemia (Hb SS) - among top scoring conditions 1, 3
  • Other hemoglobinopathies - included in screening panels 3

• Other conditions:

  • Cystic fibrosis - added to many state screening programs 3
  • Congenital hearing loss - point-of-care test rather than blood spot analysis 3

Screening Technology and Methods

The expansion of newborn screening has been largely enabled by technological advances:

• Tandem mass spectrometry (MS/MS) - allows for detection of multiple metabolic disorders from a single dried blood spot sample 1, 6
• Dried blood spot analysis - collected from heel pricks, typically before hospital discharge 1, 2
• Point-of-care tests - used for conditions like hearing loss 7

Diagnostic Confirmation Process

When an abnormal screening result is detected, a specific diagnostic algorithm is followed:

1. Initial referral to specialty center for follow-up testing 1

2. Confirmatory testing which may include:

   • Repeat measurement of screening metabolites 1
   • Enzyme activity assays 1
   • Molecular genetic testing (mutation analysis) 1
   • Additional biochemical tests (urine organic acids, plasma amino acids, etc.) 1

3. Diagnostic confirmation varies by condition:

   • For some disorders (e.g., PKU), persistent abnormal metabolites may be sufficient 1
   • For others (e.g., VLCAD deficiency), enzyme assays or genetic testing are required 1
   • Biopterin synthesis defects must be ruled out even in classic PKU 1

Importance of Definitive Diagnosis

Proper diagnostic confirmation is critical for several reasons:

• Prevents incorrect or unclear diagnoses ("metabolite diagnoses") 1
• Enables appropriate treatment initiation to prevent morbidity and mortality 2, 7
• Improves screening program quality by refining cut-off values and algorithms 1
• Reduces false positives over time while maintaining low false-negative rates 1
• Distinguishes true disease from benign variants or polymorphisms 1

Expansion of Newborn Screening

The Secretary of the U.S. Department of Health and Human Services adopted recommendations for a uniform screening panel that includes:

• 30 core conditions - primary screening targets 1
• 26 secondary conditions - disorders that may be detected during screening for core conditions 1

This expansion has presented challenges to ensuring quality performance in both public health laboratories and diagnostic laboratories that perform confirmatory testing 1.

Pitfalls and Considerations

• Timing of sample collection can affect results - samples collected too early may miss some conditions 3
• Prematurity, diet, transfusions, and total parenteral nutrition can affect screening results 3
• Metabolite diagnoses occur when mildly abnormal metabolites persist but definitive testing is not completed, leading to diagnostic uncertainty 1
• Secondary targets often have less understood natural histories and may include clinically asymptomatic individuals with biochemical abnormalities 1
• False positives can cause parental anxiety and unnecessary medical evaluations 1, 4

By following established diagnostic guidelines and ensuring proper confirmatory testing, these potential pitfalls can be minimized while maximizing the life-saving benefits of newborn screening.