Neonatal Screening for Metabolic Diseases

1. Introduction and Role of Newborn Screening

Newborn screening (NBS) is a comprehensive public health strategy aimed at the early identification of infants affected by certain genetic, metabolic, hormonal, and functional conditions.

• Primary Role: Secondary prevention. It aims to detect disorders presymptomatically (before clinical signs appear) to initiate timely intervention.
• Public Health Impact:
  • Reduction of Mortality: Prevents neonatal and infant deaths caused by metabolic crises (e.g., salt wasting in CAH, hyperammonemia in UCDs).
  • Prevention of Morbidity: Prevents irreversible sequelae such as intellectual disability (e.g., in Congenital Hypothyroidism, PKU), physical disability, and growth failure.
  • Genetic Counseling: Identifies index cases, allowing for family screening and prenatal diagnosis for future pregnancies.
  • Epidemiological Data: Helps estimate the true burden of IEMs in the population (e.g., Indian incidence of CH is ~1:1172, significantly higher than global averages).

2. Criteria for Selection: In Which Conditions is NBS Useful?

Disorders selected for NBS must generally satisfy the Wilson and Jungner Criteria:

1. Important Health Problem: The condition should be frequent or have severe consequences (high mortality/morbidity).
2. Latent Stage: There must be a recognizable asymptomatic phase.
3. Natural History: The disease progression must be well understood.
4. Effective Treatment: Treatment must be available and more effective if started early.
5. Reliable Test: A suitable, acceptable, and accurate screening test must be available.

A. The "Core Panel" (Recommended for India)

Based on prevalence and treatability in the Indian context (NNF/ICMR recommendations):

1. Congenital Hypothyroidism (CH):
   • Incidence: 1:1000 – 1:1500 (High in India).
   • Utility: Early thyroxine therapy prevents profound intellectual disability ("Cretinism").
2. Congenital Adrenal Hyperplasia (CAH):
   • Incidence: ~1:5700.
   • Utility: Prevents life-threatening salt-wasting crises and incorrect gender assignment in females.
3. Glucose-6-Phosphate Dehydrogenase Deficiency (G6PD):
   • Utility: Prevents severe neonatal jaundice, kernicterus, and hemolysis by avoiding triggers (oxidative drugs, fava beans).

B. Expanded Screening (Inborn Errors of Metabolism - IEM)

With the advent of Tandem Mass Spectrometry (LC-MS/MS), 40+ disorders can be screened from a single blood spot.

• Amino Acid Disorders: Phenylketonuria (PKU), Maple Syrup Urine Disease (MSUD), Homocystinuria, Tyrosinemia.
• Fatty Acid Oxidation Disorders (FAODs): MCAD deficiency, VLCAD deficiency. Important for preventing sudden infant death.
• Organic Acidemias (OAs): Methylmalonic acidemia, Propionic acidemia, Isovaleric acidemia, Glutaric Aciduria Type 1.
• Other Defects: Galactosemia (GALT deficiency), Biotinidase Deficiency, Cystic Fibrosis.

3. Methodology: How It Is Done

NBS is a system, not just a test. The workflow involves several critical steps:

A. Pre-Analytical Phase (Sampling)

• Timing:
  • Ideal: Between 48 to 72 hours of life.
  • Reasoning: Allows for stabilization of TSH (post-birth surge) and ensures the baby has received protein feeds (essential for detecting PKU/amino acidopathies).
  • Pre-discharge: If discharged <24 hours, a sample should be taken, but a repeat sample is often required at 1–2 weeks.
• Collection Method:
  • Heel Prick: Blood is collected from the plantar surface of the heel (medial or lateral aspect to avoid calcaneus).
  • Dried Blood Spot (DBS): Drops are applied to a specific filter paper (Whatman 903/Guthrie card).
  • Drying: Air-dried horizontally for at least 3-4 hours away from sunlight/heat.
  • Transport: Placed in an envelope (with desiccant if humid) and couriered to the central laboratory within 24 hours.

B. Analytical Phase (Laboratory Testing)

Different technologies are used based on the analyte:

1. Immunoassays (ELISA / DELFIA): * Used for: TSH (CH), 17-OHP (CAH), IRT (Cystic Fibrosis).
   • DELFIA (Dissociation-Enhanced Lanthanide Fluorescent Immunoassay) is the gold standard due to high sensitivity.
2. Tandem Mass Spectrometry (LC-MS/MS):
   • Used for: Amino acids (PKU, MSUD) and Acylcarnitines (FAODs, OAs).
   • Mechanism: Weighs molecules to identify abnormal metabolite patterns. High throughput, multiplexing capability.
3. Enzymatic/Colorimetric Assays:
   • Used for: Galactosemia (GALT activity), G6PD deficiency, Biotinidase.

C. Post-Analytical Phase (Reporting & Follow-up)

• Recall: Notification of parents/pediatricians immediately for "Screen Positive" results.
• Confirmatory Testing: A fresh sample (venous blood/urine) is sent for diagnostic tests (e.g., serum TSH/T4, plasma amino acids, urine organic acids, gene sequencing).

4. Interpretation of NBS Results

Interpretation requires clinical correlation and understanding of physiology.

A. Result Categories

1. Screen Negative (Normal): Probability of disease is low. Note: Does not completely rule out mild variants or errors.
2. Screen Positive (Abnormal): The analyte value is outside the established cut-off (e.g., TSH > 20 mIU/L).
   • Action: Requires immediate follow-up. This is NOT a diagnosis; it indicates high risk.
3. Borderline: Value is marginally elevated. usually requires a repeat DBS sample.

B. Factors Affecting Interpretation (Potential False Positives/Negatives)

• Prematurity/LBW:
  • CH: Delayed TSH rise (hypothalamic immaturity).
  • CAH: 17-OHP is naturally higher in preterms (stress response), causing false positives. Use weight-stratified cut-offs.
  • Tyrosinemia: Transient tyrosinemia of newborn is common.
• Timing of Sample:
  • <24 hours: High TSH surge (false positive for CH).
  • Before feeds: False negative for PKU/Galactosemia (metabolites haven't accumulated).
• Maternal Factors:
  • Maternal steroid intake (suppresses fetal 17-OHP).
  • Maternal thyroid antibodies.
• Transfusion: Can mask G6PD deficiency or Galactosemia (donor cells are normal). Sample should be taken before transfusion or 3-4 months later.

C. Algorithm for Abnormal Results

1. Critical Value (e.g., extremely high Ammonia/Acylcarnitine): Immediate admission, stop feeds, start glucose/IVF, collect confirmatory sample.
2. Elevated Value (Asymptomatic): Recall for repeat DBS or confirmatory serum testing.
3. Confirmation:
   • CH: Serum T3, T4, TSH.
   • CAH: Serum 17-OHP, Electrolytes.
   • IEM: TMS (plasma), GC-MS (urine), Genetic testing (Next-Gen Sequencing).

5. Summary of Key NBS Conditions & Markers