12.2 Genetic Testing in the NICU | Neonatology Academy

Bedside Priority Statement

Four rules for NICU genetic testing

Genetic testing should answer a clinical question; do not order a test simply because a neonate is critically ill.
Call Genetics early for infants with multiple congenital anomalies, dysmorphism with organ disease, unexplained severe hypotonia, neonatal seizures, unexplained cardiomyopathy, suspected DSD, nonimmune hydrops, persistent lactic acidosis, hyperammonemia, or a family history of a lethal neonatal disorder.
In a suspected metabolic emergency, collect critical blood/urine samples immediately and start stabilization. Do not wait for sequencing results before treating hypoglycemia, hyperammonemia, acidosis, seizures, or sepsis-like shock.
For rapid sequencing, trio testing of infant plus both biologic parents gives faster and more interpretable results than proband-only testing whenever parents are available.

1. Purpose and Scope — When Genetic Testing Matters

This chapter provides a bedside approach to genetic testing in neonates. It explains when to involve Genetics, which test is most appropriate, how to collect samples, how to communicate consent and results, and how to avoid common NICU errors. It is designed to complement, not replace, metabolic emergency protocols, newborn screening, and local laboratory requirements.

Clinical Situation	Why Genetic Testing Matters	Immediate Action
Multiple congenital anomalies, dysmorphism, suspected syndrome	A single diagnosis may explain organ involvement, prognosis, recurrence risk, and surveillance needs.	Genetics consult; consider chromosomal microarray, karyotype/FISH when appropriate, or rapid exome/genome if critically ill.
Critically ill infant with unexplained disease after routine workup	Rapid genomic diagnosis can change medications, diet, subspecialty management, procedures, transplant decisions, or goals-of-care discussions.	Discuss rapid trio exome/genome and obtain parent samples if available.
Suspected aneuploidy (trisomy 13/18/21, Turner syndrome, triploidy, mosaicism)	Chromosome structure and recurrence mechanism affect counseling and management.	Order karyotype; consider STAT FISH for rapid preliminary information when management depends on diagnosis.
Suspected DSD	Urgent sex-chromosome/SRY information may guide immediate evaluation but should not force premature sex assignment.	STAT FISH for X/Y/SRY when indicated; karyotype/CMA and endocrine workup with Genetics/Endocrinology.
Unexplained hypoglycemia, hyperammonemia, acidosis, lactic acidosis, seizures, liver failure	A metabolic disorder may be treatable and samples are most informative during crisis.	Collect critical labs, start emergency therapy, call metabolic genetics, then order targeted gene/panel/genome testing if needed.
Positive newborn screen	Screening is not diagnostic; false positives and false negatives occur.	Confirm with condition-specific biochemical and/or molecular testing; treat time-sensitive conditions immediately.

2. Choosing the Correct Genetic Test

The best test depends on the suspected mechanism: chromosome number/structure, copy-number change, single-gene sequence variant, repeat expansion, methylation/imprinting abnormality, mitochondrial disorder, or broad undifferentiated genetic disease.

Turnaround times to plan around (Baylor Ed. 33)

Chromosome analysis: 2–3 weeks. Still first-line for balanced translocations, triploidy, mosaicism and some sex chromosome abnormalities including Turner syndrome — and recommended for every patient with Down syndrome to distinguish trisomy 21 from a translocation, since that changes the parents' recurrence risk.
STAT FISH: 48–72 hours for aneuploidy, if ordered STAT and the sample reaches the laboratory during working hours on the day of collection.
Critical Trio whole exome sequencing: about 5 days, giving a specific diagnosis in roughly 40% of infants. Incidental "actionable" findings currently span 78 genes — including adult-onset conditions such as hereditary breast cancer — which is why consent and pre-test counselling are required, and why families choose in advance which categories of result they want reported.
What exome sequencing misses: trinucleotide repeat disorders such as congenital myotonic dystrophy, and copy-number changes — so pair it with high-resolution CMA. Whole genome sequencing covers both, plus deep intronic and non-coding variants; consult genetics before ordering WGS or RNA sequencing.

Test	Best NICU Use	Strengths	Important Limitations / Cautions
Karyotype / chromosome analysis	Suspected trisomy 13/18/21, Turner syndrome, triploidy, mosaicism, balanced translocation, or family counseling after aneuploidy.	Visualizes chromosome number and large structural rearrangements; detects balanced translocations that CMA cannot detect.	Lower resolution for small deletions/duplications; turnaround often days to weeks unless expedited.
STAT FISH	Rapid preliminary answer for suspected aneuploidy, sex chromosome/SRY testing in DSD, or a specific deletion when CMA is unavailable or too slow.	Can provide targeted results quickly when the right probe is selected.	Only answers the question asked; a normal FISH does not exclude other genetic disease.
Chromosomal microarray (CMA) with SNP data	First-line test for multiple congenital anomalies, dysmorphism with organ disease, unexplained developmental concern, and many syndromic presentations.	Detects pathogenic copy-number gains/losses, microdeletion/duplication syndromes, absence of heterozygosity, possible uniparental disomy, and consanguinity patterns.	Does not detect balanced rearrangements, small SNVs/indels, most repeat expansions, many low-level mosaics, or many methylation disorders; VUS may require parental studies.
Single-gene sequencing ± deletion/duplication	When phenotype or biochemical profile strongly points to one gene (e.g., OTC deficiency, CFTR, SMN1-related SMA).	Focused, interpretable, and often less expensive when clinical suspicion is high.	Poor choice when phenotype is broad or multiple genes can cause the same presentation.
Targeted multigene panel	Phenotype-driven disorders: cardiomyopathy, hypoglycemia, cholestasis, skeletal dysplasia, epilepsy, RASopathy/Noonan spectrum, renal cystic disease, or DSD.	Covers many known genes in a condition-specific area; can include deletion/duplication analysis.	Panel content varies by lab and can become outdated; may miss newly discovered genes, noncoding variants, CNVs, or repeat expansions.
Trio whole-exome sequencing (WES)	Undiagnosed infant with likely monogenic disease, multiple anomalies, severe neurologic disease, or critical illness when a targeted test is not obvious.	Interrogates coding regions across thousands of genes; trio analysis improves de novo and recessive interpretation.	May miss noncoding variants, repeat expansions, some CNVs, methylation/imprinting disorders, and structural variants; requires pretest counseling about secondary findings.
Rapid trio WES or WGS	Critically ill neonate where diagnosis may change immediate care, surgery, transplant, diet, medications, or goals of care.	Higher utility when ordered early and with parent samples; may shorten diagnostic odyssey.	Requires urgent consent, clear phenotype, and coordination with Genetics and lab; negative result does not rule out genetic disease.
Whole-genome sequencing (WGS)	Severe undiagnosed disease when broad detection is needed, especially if exome/CMA are negative or repeat/noncoding/structural disease is suspected.	More comprehensive than exome; can detect many SNVs, indels, CNVs, structural variants, some repeat expansions, mitochondrial and noncoding variants depending on platform.	Availability, coverage, turnaround, interpretation, and reporting policy vary; consult Genetics before ordering.
Methylation / imprinting tests	Suspected Prader-Willi/Angelman, Beckwith-Wiedemann, Russell-Silver, transient neonatal diabetes, or other imprinting disorders.	Detects imprinting mechanisms not detected by routine sequencing.	Often requires phenotype-specific test selection; standard CMA/WES may miss the diagnosis.
Repeat-expansion testing	Suspected congenital myotonic dystrophy or other repeat disorders.	Directly tests the mechanism that may be missed by exome and CMA.	Family history, hypotonia, respiratory failure, maternal myotonia, or polyhydramnios should prompt targeted testing.
Mitochondrial testing	Severe lactic acidosis, cardiomyopathy, liver failure, myopathy, neurologic disease, or multisystem energy failure.	May include mtDNA sequencing/deletion testing, nuclear mitochondrial panels, WES/WGS, or tissue-specific studies.	Blood can miss tissue-specific heteroplasmy; consult Genetics/Metabolism before invasive sampling.

3. Bedside Ordering Algorithm

Step	Question	Action
1	Is the infant unstable or in a metabolic crisis?	Stabilize first. Send blood gas, glucose, lactate, ammonia, electrolytes, CBC, urinalysis/ketones, plasma amino acids, acylcarnitine profile, urine organic acids, and condition-specific samples before therapy when feasible. Start emergency treatment and call metabolic genetics.
2	Is there a recognizable syndrome or aneuploidy?	Use targeted testing: karyotype for aneuploidy/structural rearrangement; STAT FISH only if rapid targeted information changes immediate care; CMA for syndromic congenital anomalies.
3	Is the phenotype strongly linked to one gene?	Order single-gene testing or a focused panel after Genetics input. Examples: SMN1 for SMA, OTC for biochemical OTC deficiency, CFTR when CF testing is indicated.
4	Is the infant critically ill with no clear diagnosis?	Consider rapid trio WES/WGS. Obtain parental samples early, document phenotype carefully, and define what result categories the family wants returned.
5	Could the mechanism be missed by exome?	Think of repeat expansions, methylation/imprinting, balanced rearrangements, mitochondrial heteroplasmy, low-level mosaicism, and deep intronic variants. Choose targeted testing or WGS/RNA sequencing with Genetics.
6	Is the first result negative or uncertain but suspicion remains high?	Re-review phenotype, request reanalysis later, check whether CNV/repeat/methylation/mitochondrial testing was included, and consider referral to a genomic medicine program.

4. Pretest Counseling and Consent in the NICU

Genetic testing can affect the infant, parents, siblings, future pregnancies, insurance planning, and long-term surveillance. For urgent tests, counseling must be practical and time-sensitive but still explicit.

Counseling Item	What the Team Should Explain
Purpose	The test is being ordered to identify a possible genetic cause of the baby's illness and to guide care, prognosis, recurrence risk, or surveillance.
Possible results	Pathogenic/likely pathogenic diagnosis; negative result; variant of uncertain significance; carrier status; incidental/secondary findings; unexpected parentage or consanguinity signal; or a result that requires parental testing.
Secondary findings	Exome/genome sequencing can identify medically actionable variants unrelated to the baby's presenting condition. Use the current ACMG secondary-finding policy and the specific laboratory's consent process.
Parental samples	Trio testing usually improves interpretation and may shorten time to diagnosis, but also can reveal parental carrier status, de novo variants, nonpaternity, or consanguinity.
Limitations	No single test finds all genetic disorders. Negative testing does not exclude a genetic diagnosis. Reanalysis may later become diagnostic as gene-disease knowledge improves.
Turnaround time	STAT FISH may result in days; karyotype often takes longer; CMA, panels, exome, and genome vary by lab and urgency. Rapid exome/genome requires special ordering workflow.
Documentation	Record who counseled the family, what result categories were discussed, whether secondary findings are requested, parent sample plan, and how results will be communicated.

5. Sample Handling: Avoid False Reassurance and Lost Opportunities

Situation	Sample / Action	Common Error to Avoid
Before transfusion when possible	Collect EDTA blood for DNA, newborn screen/DBS, and disorder-specific biochemical tests when clinically feasible.	Multiple transfusions can complicate some screening assays and interpretation.
Metabolic decompensation	Collect blood and urine during the acute episode: ammonia, lactate, blood gas, acylcarnitines, plasma amino acids, urine organic acids, ketones, reducing substances when indicated.	Waiting until the infant recovers may normalize diagnostic metabolites.
Suspected hyperammonemia	Free-flowing venous or arterial sample, placed on ice, processed urgently.	Hemolysis, delayed processing, or prolonged draw can distort results.
Suspected galactosemia / liver failure	GALT activity or galactose-1-phosphate; urine galactitol if transfused; total/direct bilirubin, coagulation studies, liver enzymes.	A negative urine reducing-substance screen does not exclude galactosemia, especially after galactose restriction.
Trio exome / genome	Infant EDTA blood plus EDTA blood or saliva/buccal sample from both biologic parents per lab instructions.	Ordering proband-only sequencing when parents are available may delay interpretation.
Postmortem or perimortem evaluation	Discuss with Genetics before death when possible: EDTA blood for DNA, frozen plasma/urine/CSF, skin biopsy in culture medium/saline, and tissue handling if indicated.	Not saving DNA/tissue may permanently prevent diagnosis and recurrence counseling.
Suspected mosaicism or tissue-limited disease	Ask Genetics whether blood is sufficient or whether skin, buccal, fibroblast, or affected tissue is needed.	A normal blood test may miss tissue-limited mosaicism.

6. Interpreting and Acting on Results

Result Type	Meaning in NICU Practice	Recommended Response
Pathogenic / likely pathogenic	The laboratory considers the variant disease-causing or probably disease-causing in the right clinical context.	Confirm phenotype fit, start disease-specific management/surveillance, communicate recurrence risk, and arrange Genetics follow-up.
Variant of uncertain significance (VUS)	Evidence is insufficient to classify the variant as disease-causing or benign.	Do not use a VUS alone for irreversible decisions. Request parental segregation, phenotype re-review, and future reanalysis.
Negative result	No reportable diagnosis was found by that test.	Reassess whether the test could detect the suspected mechanism; consider repeat-expansion, methylation, mitochondrial, structural, RNA, or tissue-specific testing if suspicion remains high.
Carrier status	The infant carries one variant in a recessive gene or an X-linked context without clear disease expression.	Usually does not explain critical illness alone, but may matter for parents, siblings, and future pregnancies.
Secondary finding	Medically actionable finding unrelated to the neonatal presentation, usually from exome/genome sequencing.	Manage according to current ACMG/laboratory policy, document consent, and refer family for genetic counseling.
Unexpected relationship / consanguinity signal	Trio or SNP-based testing may show nonpaternity, consanguinity, or absence of heterozygosity.	Handle confidentially; involve Genetics, genetic counseling, ethics, and institutional policy when needed.

Do not over-interpret genetic results

A VUS is not a diagnosis unless future evidence changes its classification.
A negative exome does not exclude a genetic disease; exome may miss repeat expansions, methylation disorders, balanced rearrangements, many noncoding variants, and tissue-limited mosaicism.
A chromosomal microarray can detect copy-number changes but cannot detect most point mutations or balanced translocations.
A STAT FISH result is preliminary and targeted; it does not replace a complete genetic evaluation.

7. Practical NICU Indication Pathways

Phenotype	First Considerations	Genetic Testing Pathway
Multiple anomalies or dysmorphism	Detailed exam, photographs per institutional policy, echocardiogram, renal ultrasound, ophthalmology/hearing when indicated.	CMA as baseline; karyotype/FISH if aneuploidy or balanced rearrangement suspected; rapid WES/WGS if critically ill or CMA unlikely to answer.
Severe neonatal hypotonia	Rule out sepsis, HIE, electrolyte/glucose problems, neuromuscular disease, congenital myopathy, SMA, Prader-Willi, metabolic disease.	SMN1 testing if SMA suspected; methylation for Prader-Willi when appropriate; CMA/WES/WGS or neuromuscular panel after Genetics/Neurology input.
Neonatal seizures / epileptic encephalopathy	EEG, glucose/electrolytes/Ca/Mg, infection, HIE/stroke/bleed, ammonia/lactate/metabolic screen.	Epilepsy gene panel or rapid WES/WGS; consider treatable conditions: pyridoxine/pyridoxal phosphate-responsive epilepsy, biotinidase deficiency, GLUT1, NKHG, sulfite oxidase/molybdenum cofactor disorders.
Cardiomyopathy / arrhythmia	Echo, ECG, electrolytes, acylcarnitines, lactate, maternal history, family sudden death.	Cardiomyopathy/arrhythmia panel, mitochondrial/FAOD evaluation, or rapid WES/WGS if critically ill; consider secondary findings policy when broad sequencing.
Cholestasis / liver failure	Sepsis, TORCH/viral disease, biliary atresia, metabolic disease, galactosemia, tyrosinemia, mitochondrial disease.	Metabolic testing plus cholestasis/liver failure panel or WES/WGS; do not delay galactose restriction if galactosemia suspected.
DSD or ambiguous genitalia	Electrolyte/glucose safety, CAH evaluation, pelvic/abdominal ultrasound, endocrine consult, careful communication.	STAT sex chromosome/SRY FISH if needed; karyotype/CMA; targeted DSD panel/WES depending on phenotype.
Nonimmune hydrops	Cardiac, infectious, hematologic, placental, lymphatic, chromosomal, and metabolic etiologies.	Karyotype/CMA, targeted testing based on exam, and early WES/WGS when unexplained or recurrent.
Positive newborn screen	Confirmatory biochemical testing according to condition; assess symptoms immediately.	Molecular confirmation when recommended by the newborn-screening program or genetics/metabolic team.

8. Quality and Safety Checklist Before Ordering

Checklist Question	Why It Matters
What is the phenotype in one sentence?	The laboratory's interpretation depends heavily on accurate phenotype terms and clinical context.
What test mechanism is needed?	Avoids ordering exome when repeat/methylation/karyotype is the correct test, or CMA when single-gene sequencing is needed.
Will the result change care now?	Supports rapid testing for critically ill infants and avoids low-value testing when immediate management will not change.
Were both parents sampled?	Trio testing improves diagnosis, clarifies inheritance, and helps classify VUS/de novo variants.
Was consent documented?	Required for exome/genome testing, secondary findings, parental samples, and return-of-result preferences.
Were urgent metabolic samples collected before therapy?	Some diagnostic markers disappear after dextrose, dialysis, protein restriction, transfusion, or clinical recovery.
Is there a plan for results disclosure?	Families need Genetics-led explanation, recurrence-risk counseling, and long-term follow-up.
Will a negative result be re-evaluated later?	Genomic reanalysis can become diagnostic as gene-disease knowledge changes.

9. Source-Difference Notes

Source	How It Was Used in This Chapter
Baylor 2025–2026	Primary structure for karyotype, FISH, CMA, single-gene testing, panels, WES, WGS, newborn-screening context, and the role of Genetics consultation.
West Midlands 2025–2028	Practical emergency-metabolic approach, recognition triggers, initial and specific biochemical investigations, urgent specialist contact, and sample handling during acute illness.
Belize 2018–2021	Supportive material from hypoglycemia, cholestasis, seizures, and NEC differential sections where IEM/genetic etiologies should be considered.
ACMG / rapid-genomics literature	Updated framing for exome/genome as first- or second-tier testing in pediatric congenital anomalies/developmental disorders, ACMG SF v3.2 secondary findings policy, and NICU rapid sequencing utility.

Key Takeaways — Chapter 12.2

Genetic testing should answer a specific clinical question — choose the test that matches the suspected mechanism, not the most available or most comprehensive option.
Call Genetics early; rapid genomic diagnosis in a critically ill infant can directly change medications, diet, procedures, or goals-of-care decisions.
In a metabolic emergency, stabilize and collect diagnostic samples first — do not wait for sequencing results before treating hypoglycemia, hyperammonemia, or acidosis.
Trio testing (infant + both parents) is the standard for rapid WES/WGS; proband-only testing is a common, avoidable error when parents are available.
CMA detects copy-number changes but misses point mutations and balanced translocations; exome/WGS misses repeat expansions, methylation disorders, and tissue-limited mosaicism.
A VUS is not a diagnosis; do not use it alone for irreversible clinical decisions — request parental segregation and future reanalysis.
A negative exome does not exclude a genetic disease. When suspicion remains high, re-examine the phenotype and consider additional test modalities.
Save perimortem/postmortem samples whenever possible — DNA/tissue not collected at the time of death cannot be retrieved later for recurrence counseling.

Selected References

Baylor College of Medicine. Guidelines for Acute Care of the Neonate, Edition 33, 2025–2026. Section 7, Chapter 7.2: Genetic Testing.
West Midlands Neonatal Operational Delivery Network. Neonatal Guidelines 2025–2028. Inherited metabolic disorders guideline.
Belize Ministry of Health. Neonatal Clinical Practice Guidelines 2018–2021. Hypoglycemia, cholestasis, NEC differential, and neonatal seizure sections.
American College of Medical Genetics and Genomics (ACMG). Evidence-based clinical practice guidelines, including exome and genome sequencing for pediatric patients with congenital anomalies or intellectual disability.
Miller DT, Lee K, Abul-Husn NS, et al. ACMG SF v3.2 list for reporting of secondary findings in clinical exome and genome sequencing. Genetics in Medicine. 2023.
Muriello M, Basel D. Rapid Exome and Genome Sequencing in the Intensive Care Unit. Critical Care Clinics. 2022.
Richards S, Aziz N, Bale S, et al. Standards and guidelines for the interpretation of sequence variants: ACMG/AMP consensus recommendation. Genetics in Medicine. 2015.

Implementation note

This chapter is for education and workflow design. Local Genetics, Metabolism, laboratory, ethics, and consent policies must determine the final test name, specimen type, consent language, and return-of-results process.