A 2-day-old neonate born to a 38-year-old primigravida presents with hypotonia, a flat facial profile, upslanting palpebral fissures, and a single palmar crease. Clinical suspicion for Down syndrome is high. Which investigation is the gold standard for definitive diagnosis?

Explanation

## Investigation of Choice for Suspected Down Syndrome ### Gold Standard: Karyotyping **Key Point:** Karyotyping on peripheral blood lymphocytes is the gold standard and definitive diagnostic test for Down syndrome (trisomy 21). It visualizes all 46 chromosomes and can detect the extra chromosome 21, as well as identify variant forms (translocation, mosaic). ### Why Karyotyping is Superior | Investigation | Sensitivity | Specificity | Detects Variants | Gold Standard? | |---|---|---|---|---| | **Karyotyping** | 100% | 100% | Yes (translocation, mosaic) | **YES** | | FISH | 95–99% | High | Limited | No | | QF-PCR | 99% | High | No | No | | Cardiac ultrasound | — | — | N/A (structural finding) | No | ### Timing & Clinical Context **High-Yield:** Karyotyping takes 7–14 days because it requires cell culture and metaphase arrest. In a symptomatic newborn with classic clinical features, it remains the reference standard despite the wait. **Clinical Pearl:** FISH and QF-PCR are rapid (24–48 hours) and useful for prenatal screening or when results are urgently needed, but they do NOT replace karyotyping for definitive diagnosis or detection of balanced translocations (which occur in ~3–4% of Down syndrome cases). ### Why Cardiac Ultrasound Is Not Diagnostic Cardiac defects (AVSD, VSD, PDA) are common in Down syndrome but are **associated findings, not diagnostic tests**. They confirm complications, not the chromosomal diagnosis. [cite:Park 26e Ch 11]