Indications for Karyotype Testing
Karyotype testing is indicated for any child with unexplained developmental delay or intellectual disability, even without dysmorphic features, and should be performed at minimum 550-band resolution 1.
Primary Pediatric Indications
Developmental and Cognitive Concerns
- Unexplained mental retardation/developmental delay (MR/DD): This is the most fundamental indication, regardless of whether dysmorphic features, other clinical findings, or positive family history are present 1
- The absence of obvious physical abnormalities does NOT exclude the need for testing
Suspected Chromosomal Syndromes
- Clinical features suggesting known chromosomal abnormalities (e.g., Down syndrome): Even when the diagnosis seems clinically obvious, karyotyping is essential because identifying a translocation versus standard trisomy dramatically affects family recurrence risk 1
Newborns and Infants
- Dysmorphic features and/or multiple congenital malformations 1, 2
- Congenital hypotonia or hypertonia 2
- Unexplained intrauterine growth retardation 2
- Single major anomaly OR multiple major/minor anomalies 2
Additional Pediatric Indications
- Failure to thrive 2
- Unusual growth patterns: overgrowth, short stature, or hemihypertrophy 2
- Six or more café-au-lait macules ≥0.5 cm diameter 2
- Bilateral or multifocal malignancies (retinoblastoma, Wilms tumor) 2
Prenatal Indications
High-Risk Screening Results
- High-risk noninvasive prenatal testing (NIPT): This carries the highest detection rate for abnormal karyotypes (36.0%) among all indications 3
- Abnormal maternal serum screening 4, 3
- Advanced maternal age (AMA): Now the most common prenatal diagnostic indication, accounting for 54.50% of cases 4
Ultrasound Findings
- Increased nuchal translucency 5
- Fetal structural anomalies on ultrasound: Particularly important in late pregnancy (24-32 weeks), where this becomes the top indication (28.6%) 6
Parental Factors
- Chromosomal abnormalities in one or both parents: Second highest detection rate (16.1%) for abnormal karyotypes 3
Critical Clinical Scenarios Requiring Karyotype Despite Normal Prenatal Testing
A blood karyotype must be performed postnatally even when fetal karyotype from amniocentesis or CVS was normal if the child has developmental delay, MR, physical anomalies, or dysmorphic features 1. Three key reasons justify this:
- Mosaicism may have been undetected in fetal tissue
- Higher banding resolution is achievable in postnatal blood preparations versus cultured amniocytes/chorionic villi
- Clinical features after birth may suggest specific chromosomal imbalances not suspected prenatally
Special Considerations and Testing Algorithm
When Standard Karyotype is Normal
If initial 550-band karyotype is normal but clinical suspicion remains high:
- Subtelomere FISH testing should be considered 1
- Skin fibroblast culture for individuals with asymmetry or hypo/hyperpigmented regions (tissue-limited mosaicism) 1
- Fragile X DNA analysis is indicated for all developmentally delayed individuals without apparent underlying cause 1
When Microdeletion/Microduplication Suspected
FISH or molecular techniques should be performed prior to or concurrently with chromosome analysis when clinical features suggest specific syndromes 1
High-Resolution Karyotyping
Not routinely indicated unless investigating a specific chromosomal region or there is family history of particular abnormality; should be limited in focus and used only when FISH unavailable 1
Common Pitfalls to Avoid
- Do not skip karyotyping in developmentally delayed children who lack dysmorphic features—the guideline explicitly states testing is indicated even in their absence
- Do not assume normal prenatal karyotype excludes postnatal testing when clinical features warrant evaluation
- Do not rely solely on NIPT: While highly accurate, it cannot detect mosaicism, structural abnormalities, or triploidy that karyotyping identifies 4
- Avoid iodine-containing compounds when cleansing skin for biopsy—use alcohol only, as iodine inhibits cell growth in culture 1
Parental Testing After Abnormal Results
When structural abnormalities are detected, both parents must undergo chromosome analysis to determine if either carries a balanced rearrangement, which clarifies the nature of the abnormality and recurrence risk 1