A 28-year-old primigravida at 34 weeks of gestation presents with a history of transfusion with unmatched blood during a motor vehicle accident 10 years ago. She is Rh-negative and her partner is Rh-positive. Antenatal screening shows an indirect Coombs test (ICT) positive at 1:16 dilution. Which investigation is most appropriate to assess the severity of fetal hemolytic disease and guide management?

Explanation

## Investigation of Choice for Severity Assessment in Rh Isoimmunisation ### Clinical Context This patient has **confirmed Rh alloimmunisation** (positive ICT) with significant antibody titre (1:16), placing the fetus at risk of haemolytic disease of the newborn (HDN). The investigation must quantify fetal anaemia to guide delivery timing and intrauterine transfusion (IUT) decisions. ### Why MCA Peak Systolic Velocity (PSV) is Correct **Key Point:** MCA-PSV by Doppler ultrasound is the **non-invasive gold standard** for detecting fetal anaemia in Rh disease from 18 weeks onwards. - **Mechanism:** Fetal anaemia → increased cardiac output and blood viscosity → increased MCA flow velocity. - **Threshold:** MCA-PSV >1.5 multiples of median (MoM) for gestational age predicts moderate-to-severe anaemia (Hb <70 g/L) with >90% sensitivity. - **Advantages:** - Non-invasive, repeatable, no risk of pregnancy loss. - Can be performed serially to monitor progression. - Guides timing of delivery or IUT without procedural risk. - Preferred initial investigation in modern practice (post-2000 evidence). ### Comparison of Investigations in Rh Isoimmunisation | Investigation | Timing | Use Case | Limitation | |---|---|---|---| | **MCA-PSV** | ≥18 weeks | First-line for severity assessment | Operator-dependent | | **Amniocentesis + Liley** | 14–35 weeks | Historical reference; rarely used now | Invasive, risk of sensitisation, labour induction | | **Cordocentesis** | ≥18 weeks | Fetal blood sampling for Hb, direct transfusion | Invasive, 1–2% loss risk; reserved for IUT | | **Kleihauer-Betke** | Antenatal/postnatal | Quantify fetal–maternal haemorrhage (FMH) | Does not assess fetal anaemia severity | **High-Yield:** In current NEET PG practice, **MCA-PSV has replaced amniocentesis** as the primary investigation for assessing fetal anaemia in Rh disease. Amniocentesis is now reserved for cases where MCA-PSV is inconclusive or unavailable. ### Clinical Pearl Once MCA-PSV indicates moderate-to-severe anaemia (>1.5 MoM), the next step is **cordocentesis for IUT**, not amniocentesis. This patient at 34 weeks with high antibody titre and positive ICT should have MCA-PSV performed urgently; if >1.5 MoM, cordocentesis and IUT are indicated. **Warning:** Do not confuse the **diagnostic** test (ICT, antibody titre) with the **severity assessment** test (MCA-PSV). The ICT confirms alloimmunisation; MCA-PSV quantifies fetal risk.