## Confirmatory Investigation After Abnormal Prenatal Screening **Key Point:** After an abnormal triple/quad screen, the definitive confirmatory investigation is **amniocentesis with karyotyping and/or FISH on amniotic fluid cells**. This is the gold standard for diagnosing chromosomal aneuploidies such as trisomy 21, providing 100% sensitivity and specificity. NIPT is a highly accurate *screening* test but is NOT a diagnostic/confirmatory test — a positive NIPT still requires invasive confirmation before clinical decisions are made. ### Diagnostic Algorithm for Abnormal Prenatal Screening | Step | Investigation | Purpose | |---|---|---| | 1 | Triple/Quad Screen | Initial screening | | 2 | NIPT (optional intermediate step) | Refined risk stratification | | 3 | **Amniocentesis + Karyotyping/FISH** | **Definitive diagnosis (gold standard)** | ### Why Amniocentesis with Karyotyping/FISH is the Correct Answer 1. **Gold standard** — Karyotyping of amniotic fluid cells provides a definitive chromosomal diagnosis with 100% sensitivity and specificity for trisomy 21. 2. **Confirmatory, not screening** — The question asks for the investigation to *confirm* the diagnosis. NIPT is a screening test; it cannot confirm a diagnosis. 3. **FISH** — Fluorescence in situ hybridization on amniotic fluid cells provides rapid (24–48 hour) results for common aneuploidies (chromosomes 13, 18, 21, X, Y), complementing full karyotyping. 4. **Standard of care for confirmation** — ACOG, RCOG, and Indian guidelines (FOGSI) all state that invasive testing (amniocentesis or CVS) is required for definitive prenatal diagnosis of chromosomal abnormalities. 5. **Timing** — At 28 weeks gestation (as in this stem), amniocentesis is entirely feasible and appropriate. ### Why the Other Options Are Incorrect - **B) NIPT** — NIPT is a *screening* test, not a confirmatory/diagnostic test. A positive NIPT result must be followed by invasive testing (amniocentesis) before a definitive diagnosis is made. NIPT cannot replace karyotyping for confirmation. - **C) Fetal echocardiography** — Detects cardiac anomalies (e.g., AVSD in Down syndrome) but does NOT confirm the chromosomal diagnosis. It is a secondary investigation after chromosomal confirmation. - **D) Quad screen** — Adding inhibin A to the triple screen improves detection rate but remains a *screening* test. It cannot confirm trisomy 21. ### Comparison of Investigations | Investigation | Type | Sensitivity | Specificity | Miscarriage Risk | Confirms Diagnosis? | |---|---|---|---|---|---| | **Amniocentesis + Karyotyping/FISH** | Diagnostic | 100% | 100% | 0.1–0.3% | **Yes** | | NIPT (cell-free fetal DNA) | Screening | >99% | >99% | None | No | | CVS + Karyotyping | Diagnostic | 100% | 100% | 0.2–0.5% | Yes | | Quad screen | Screening | ~80% | ~95% | None | No | **High-Yield:** The critical distinction in prenatal genetics is **screening vs. diagnosis**. Triple/quad screen and NIPT are *screening* tests that stratify risk. Amniocentesis with karyotyping/FISH is the *diagnostic* gold standard that *confirms* trisomy 21. This distinction is heavily tested in NEET PG / INI-CET. **Clinical Pearl:** In Indian practice (FOGSI/ICMR guidelines), amniocentesis with karyotyping remains the standard confirmatory test for chromosomal aneuploidies. NIPT, while increasingly available, is still classified as an advanced screening tool and does not replace invasive diagnostic testing for confirmation. **Mnemonic:** **CONFIRM = Chromosomal analysis On amniotic fluid Needed For Invasive Reliable Molecular diagnosis** [cite: Williams Obstetrics 25e Ch 14; ACOG Practice Bulletin 226; FOGSI Guidelines on Prenatal Diagnosis] 
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