Newborn Screening

Newborn screening tests a small blood sample from a heel prick, collected on filter paper (dried blood spot) at 24 to 72 hours of age (and after 48 hours in preterm or sick infants). It screens for disorders that are treatable and cause irreversible harm if recognised late.

Components differ by programme and include:

• Biochemical analysis by tandem mass spectrometry (amino acid and acylcarnitine profiles for phenylketonuria, maple syrup urine disease, urea cycle defects, fatty acid oxidation defects, organic acidaemias).
• Immunoassays (thyroid stimulating hormone for congenital hypothyroidism, 17-hydroxyprogesterone for congenital adrenal hyperplasia, immunoreactive trypsinogen for cystic fibrosis).
• Enzyme assays (biotinidase, galactose-1-phosphate uridyltransferase, lysosomal enzymes).
• Haemoglobin analysis (isoelectric focusing or HPLC for sickle cell disease and thalassaemia).
• Molecular tests (T-cell receptor excision circles for severe combined immunodeficiency, SMN1 deletion for spinal muscular atrophy, CFTR variants).
• Point-of-care screens (pulse oximetry for critical congenital heart disease, hearing screening by otoacoustic emissions).

Programmes screen 30 to 60 or more conditions, and detect about 1 in 800 to 1 in 1000 newborns with a screened disorder. Approximate rates:

• Hypothyroidism: about 1 in 2000.
• Phenylketonuria: 1 in 10 000 to 15 000.
• Cystic fibrosis: 1 in 3000 to 4000 in white populations.
• Sickle cell disease: 1 in 400 to 1 in 700 in parts of Africa.

Early treatment prevents intellectual disability (phenylketonuria, congenital hypothyroidism), adrenal crisis and death (congenital adrenal hyperplasia, fatty acid oxidation defects), sepsis and death (sickle cell disease, with penicillin prophylaxis), and infection (severe combined immunodeficiency, with transplant).

Components. Congenital hypothyroidism (thyroid stimulating hormone). Congenital adrenal hyperplasia (17-hydroxyprogesterone). Phenylketonuria and other amino acid disorders. Fatty acid oxidation defects (MCAD) and organic acidaemias (acylcarnitines). Galactosaemia. Biotinidase deficiency. Cystic fibrosis. Haemoglobinopathies (sickle cell disease, thalassaemia).

Severe combined immunodeficiency. Spinal muscular atrophy. Critical congenital heart disease (pulse oximetry). Hearing loss. Congenital cytomegalovirus and others where programmes include them.

Indications. Every newborn, regardless of health or family history, with informed parental consent (or opt out) as the programme requires. Repeat sampling for early (under 24 hours) collection, transfusion, parenteral nutrition, prematurity, antibiotics or insufficient sample. Not a diagnostic test: positive screens need confirmatory testing.

Specimen.

• Heel prick blood spot on a filter paper card (5 circles, each fully saturated through the paper, 75 to 100 microlitres).
• Collected from the side of the heel after warming, with correct technique (no squeezing, no layering, no contact with creams).
• Air dried for 3 or more hours at room temperature away from heat and sunlight, and posted to the laboratory within 24 hours.

Record gestational age, birth weight, feeding, transfusion, parenteral nutrition, medications and time of collection. Pulse oximetry on right hand and foot at 24 to 48 hours. Hearing screening before discharge.

Congenital hypothyroidism. Thyroid stimulating hormone on a dried blood spot: normal under 10 mIU/L of blood (cut off programme specific), with 10 to 20 requiring repeat and over 20 requiring urgent serum thyroid function. Confirmation: serum thyroid stimulating hormone and free thyroxine. Treatment with levothyroxine within 2 weeks gives normal IQ. Preterm infants may have delayed thyroid stimulating hormone rise and need repeat testing at 2 to 4 weeks.

Congenital adrenal hyperplasia (21-hydroxylase deficiency). 17-hydroxyprogesterone: raised in affected infants, with cut offs adjusted for gestation and birth weight because preterm and sick infants have false positives. Confirmation: serum 17-hydroxyprogesterone, electrolytes, cortisol, steroid profile and genotyping. Salt wasting crisis (vomiting, hyponatraemia, hyperkalaemia, hypoglycaemia) at 7 to 14 days.

Phenylketonuria. Phenylalanine over 120 micromol/L (2 mg/dL) with raised phenylalanine to tyrosine ratio; confirmation by plasma amino acids and pterin and dihydropteridine reductase studies (to exclude tetrahydrobiopterin defects). Dietary treatment before 10 days of life gives normal IQ.

Fatty acid oxidation defects and organic acidaemias. Acylcarnitine profile: raised C8 (octanoylcarnitine) in medium chain acyl-CoA dehydrogenase deficiency, raised C5 in isovaleric acidaemia, raised C3 in propionic and methylmalonic acidaemias; confirmation by urine organic acids, plasma acylcarnitines and gene testing.

Galactosaemia. Total galactose and galactose-1-phosphate uridyltransferase activity; confirmation by enzyme assay and genotype. Neonatal sepsis from Escherichia coli, liver failure and cataracts.

Biotinidase deficiency. Biotinidase activity low (under 10% of mean); confirmation by serum enzyme assay. Treated with biotin.

Cystic fibrosis. Immunoreactive trypsinogen raised (over the 99th percentile or 60 to 70 micrograms/L), followed by CFTR variant analysis and then sweat chloride (over 60 mmol/L diagnostic, 30 to 59 intermediate).

Haemoglobinopathies. Haemoglobin pattern: FS (sickle cell disease), FSC (haemoglobin SC disease), FA with trait (AS carrier), and thalassaemia patterns. Confirmation by repeat analysis and DNA studies. Penicillin prophylaxis from 2 months.

Severe combined immunodeficiency. T-cell receptor excision circles low (under 25 to 30 in each microlitre of blood): confirm with lymphocyte subsets; avoid live vaccines, isolate, and refer for transplant.

Spinal muscular atrophy. Homozygous deletion of SMN1 exon 7: referral to neurology and early gene therapy, antisense oligonucleotide or oral splicing modifier.

Critical congenital heart disease. Pulse oximetry: saturation under 90% at any time, or under 95% in both limbs on 3 measures, or over 3% difference between right hand and foot: echocardiography.

Interpretation.

• Positive screen: not a diagnosis. Programmes have positive predictive values of 5 to 50% depending on the condition, with false positives from prematurity, early collection, stress and treatment.
• Negative screen does not exclude a condition (false negatives occur: 1 to 5% in some disorders), and symptoms should lead to diagnostic testing regardless of newborn screening results.
• Borderline results need repeat samples within days, with clinical review.
• Carrier detection (haemoglobin trait, cystic fibrosis carrier) is incidental and parents need counselling.
• Results are returned in 3 to 7 days for urgent conditions, and the programme contacts families for positive results.

Critical values and findings needing immediate action.

• Raised 17-hydroxyprogesterone with ambiguous genitalia, vomiting or hyponatraemia: salt wasting crisis, intravenous fluids, hydrocortisone and endocrine review the same day.
• Markedly raised phenylalanine, galactose or leucine, or abnormal acylcarnitines with lethargy, vomiting, poor feeding or seizures: metabolic emergency, stop feeds, intravenous glucose, ammonia, blood gas and specialist metabolic team.
• Thyroid stimulating hormone over 40 mIU/L: urgent confirmation and levothyroxine within days.
• Fatty acid oxidation defect: avoid fasting, give frequent feeds and glucose, and emergency regimen during illness.
• Absent T-cell receptor excision circles: protective isolation, avoid live vaccines (rotavirus, BCG), irradiated blood products, and early transplant.
• Sickle cell disease: penicillin, vaccinations (pneumococcal), parental education about fever and splenic sequestration.
• Failed pulse oximetry: urgent echocardiography.

Interferences and pitfalls.

• Sampling before 24 hours gives false positives for thyroid stimulating hormone and metabolic markers, and sampling after transfusion makes haemoglobin and enzyme assays unreliable (repeat 120 days after transfusion).
• Parenteral nutrition raises amino acids; antibiotics and steroids alter 17-hydroxyprogesterone; prematurity causes delayed thyroid stimulating hormone rise and false 17-hydroxyprogesterone elevation.
• Poor specimen quality (layering, contamination, insufficient saturation, incorrect drying) is the commonest cause of repeat requests.
• Missed or delayed samples for infants born at home, transferred, or from marginalised families; tracking systems are needed.
• Parents may decline screening, and the programme documents informed refusal.
• Incidental findings (carrier status, unrelated disorders) and storage of dried blood spots raise privacy issues.
• Genomic newborn screening is under evaluation and brings questions about variants of uncertain significance and adult onset conditions.
• Programmes differ greatly between countries; many low income countries screen only selected conditions or lack national screening, so early diagnosis depends on clinical awareness.
• Screening, confirmation, treatment and long term follow up must be linked; a positive result without access to treatment does not help the infant.

Special groups.

• Preterm and low-birth-weight infants: repeat screening at 28 days or discharge, with gestation specific cut offs.
• Sick infants in neonatal units: transfusion and nutrition effects, with documented repeat testing.
• Infants of mothers with thyroid, adrenal or metabolic disease: maternal disease causes false results (maternal thyroid antibodies, maternal phenylketonuria).
• Home births and migrants: ensure access and consent.
• Families with a history of an affected child: diagnostic testing rather than screening.
• Regions: sickle cell disease screening is a priority in sub-Saharan Africa and India, where over 300 000 affected infants are born each year, and the programme with prophylaxis and education lowers under-5 mortality.

Follow up tests. Repeat dried blood spot, serum thyroid function, serum 17-hydroxyprogesterone with electrolytes, plasma amino acids, urine organic acids and plasma acylcarnitines, ammonia and blood gas, galactose-1-phosphate uridyltransferase assay, sweat chloride and CFTR sequencing, haemoglobin electrophoresis or HPLC with DNA analysis,

lymphocyte subsets and genetic testing for immunodeficiency, SMN1 and SMN2 copy number, echocardiography, audiology, and genetic counselling for families.