What laboratory workup is recommended for evaluating a child with developmental delay?

Laboratory Workup for Developmental Delay

For a child with developmental delay of unknown etiology, order chromosomal microarray (CMA) and Fragile X testing as first-line laboratory tests, along with hearing and vision screening. 1

Initial Clinical Assessment

Before ordering any laboratory tests, perform:

• Comprehensive history: Focus on prenatal exposures, birth complications, developmental milestones, regression, seizures, and three-generation family pedigree looking for consanguinity, unexplained deaths, or intellectual disability
• Physical examination: Specifically assess for dysmorphic features, head circumference (macrocephaly/microcephaly), neurocutaneous findings (use Wood's lamp), organomegaly, and neurological abnormalities
• Hearing and vision screening: Must be completed before formal developmental testing 1

First-Tier Laboratory Testing

The most recent guidelines (2020-2021) consistently recommend:

Universal First-Line Tests

• Chromosomal microarray (CMA): Replaces standard karyotype and FISH subtelomere testing 1, 2
• Fragile X molecular testing: Recommended for all children with unexplained developmental delay 1, 2

Conditional First-Line Tests

• Metabolic testing: Only if suggested by history/physical examination findings such as:
  • Regression or episodic decompensation
  • Unexplained seizures
  • Dysmorphic features suggesting metabolic disorder
  • Consanguinity
  • Specific metabolic indicators (unusual odor, organomegaly, coarse features) 1

Second-Tier Testing

If first-tier tests are non-diagnostic:

• Exome sequencing (ES) or whole genome sequencing (WGS): The 2021 ACMG guideline recommends ES/WGS as a first- or second-tier test for unexplained developmental delay/intellectual disability 1
• Gene panels: Consider for specific clinical presentations
• MECP2 testing: For females with unexplained developmental delay 1
• Mitochondrial DNA testing: If clinical features suggest mitochondrial disorder 1

Neuroimaging

Brain MRI (preferred over CT) should be obtained when:

• Abnormal neurological examination
• Microcephaly or macrocephaly
• Focal neurological deficits
• Seizures
• Regression
• Specific dysmorphic features suggesting brain malformation 3, 2

MRI is not routinely indicated if examination is normal and genetic testing provides a diagnosis.

Special Circumstances

X-linked Disorder Suspected (Family History)

• X-linked intellectual disability panel
• High-density X-chromosome CMA
• Consider X-inactivation testing in mother 1

Female with Unexplained Delay

• MECP2 sequencing and deletion/duplication analysis 1

Suspected Mosaicism or Normal Blood Karyotype with High Clinical Suspicion

• Skin fibroblast culture for chromosome analysis 4

Common Pitfalls to Avoid

1. Do not order routine karyotype: CMA has replaced standard karyotype as first-line testing and detects more abnormalities 2
2. Do not order routine metabolic screening: Only order when clinical indicators are present; blanket metabolic testing has low yield 1, 3
3. Do not skip genetic testing even if prenatal karyotype was normal: Mosaicism may be present, and postnatal testing has higher resolution 4
4. Do not delay intervention while awaiting test results: Begin early intervention services immediately upon recognizing delay 3

Evolving Landscape

The most recent evidence (2021 ACMG) supports moving toward exome/genome sequencing as first-line testing rather than second-tier, as it provides higher diagnostic yield (up to 30-40% in some series) compared to CMA alone (10-15% yield) 1. However, insurance coverage and cost considerations may still necessitate the traditional stepwise approach in many practice settings.

Genetic counseling referral should be considered early, particularly for complex cases or when first-tier testing is unrevealing 1.