Which test identifies a chromosome 21 translocation?

Conventional G-banded Karyotyping is the Gold Standard Test for Detecting Chromosome 21 Translocations

Conventional G-banded chromosome analysis (karyotyping) is the primary test to identify chromosome 21 translocations, as it can detect both balanced and unbalanced rearrangements that other methods miss. 1

Why Karyotyping is Essential for Translocation Detection

The critical limitation of newer technologies is their inability to detect balanced translocations. Array-based technologies (chromosomal microarray/CMA) cannot identify balanced chromosomal rearrangements such as translocations or inversions, nor can they differentiate free trisomies from unbalanced Robertsonian translocations 1. This is a fundamental gap because:

• Approximately 5% of Down syndrome cases result from Robertsonian translocations involving chromosome 21 2
• Balanced translocation carriers are phenotypically normal but have significantly altered recurrence risks (up to 50% in some families) 1
• Identification of a translocation dramatically changes genetic counseling and family planning 3

When to Use FISH as a Complementary Test

Fluorescence in situ hybridization (FISH) serves as a powerful adjunct when:

• Confirming suspected translocations detected on karyotype 1
• Detecting cryptic translocations not visible on standard banding (minimum 550-band resolution) 3, 4, 5
• Rapid diagnosis is needed in specific clinical scenarios 6
• Parental studies are required after array detection of chromosome 21 abnormalities 1

In hematologic malignancies specifically, FISH for chromosome 21 rearrangements (such as ETV6::RUNX1 fusion in B-ALL) is performed alongside karyotyping 7, 6.

Clinical Algorithm for Translocation Detection

For suspected Down syndrome or chromosome 21 abnormalities:

1. First-line: G-banded karyotype (minimum 550-band resolution) 3

   • Detects Robertsonian translocations [rob(15;21), rob(21;21)]
   • Identifies reciprocal translocations
   • Provides structural information for recurrence risk counseling

2. Add FISH if:

   • Karyotype is normal but clinical suspicion remains high 3
   • Cryptic rearrangement is suspected based on family history 4, 5
   • Parental studies needed to determine if rearrangement is inherited 1

3. CMA is NOT appropriate for translocation detection but may be used for copy number assessment after karyotype 8

Critical Pitfall to Avoid

Never rely solely on CMA or NIPS (noninvasive prenatal screening) to rule out translocations. While CMA detects copy number changes, it provides no structural information 8. A normal CMA does not exclude a balanced translocation carrier state, which has profound implications for recurrence risk. For any child with Down syndrome, cytogenetic analysis should be performed even if the diagnosis seems clinically obvious, because identification of a translocation may affect the family's recurrence risk 3.

Confirmatory Testing in Specific Scenarios

For prenatal diagnosis following positive NIPS: Chromosome analysis on CVS or amniocentesis remains the confirmatory test 8. If translocation is suspected, metaphase FISH can provide additional structural detail 9, 8.

For cryptic translocations: FISH with chromosome 21-specific probes combined with whole chromosome paints can identify rearrangements invisible on standard karyotype 5, 10, 11. Research demonstrates that pericentromeric cryptic translocations may contribute to chromosome 21 nondisjunction 4.

The bottom line: karyotyping detects translocations; CMA does not. Always order karyotype when translocation is a consideration 1, 3.