A 28-year-old primigravida with Rh-negative blood group presents at 16 weeks gestation for routine antenatal care. She has no history of transfusion or sensitisation. Which investigation is most appropriate to assess the risk of haemolytic disease of the newborn in this pregnancy?

Explanation

## Investigation of Choice for Rh Isoimmunisation Screening ### Clinical Context This is an unsensitised Rh-negative primigravida at 16 weeks gestation. The question asks for the investigation to **assess risk** of haemolytic disease, not to diagnose it in an already-sensitised mother. ### Indirect Coombs Test (Indirect Antiglobulin Test) **Key Point:** The indirect Coombs test (IAT) detects **free antibodies in maternal serum** — specifically IgG anti-D antibodies that have not yet bound to fetal RBCs. **High-Yield:** - Performed on **maternal serum** to detect circulating antibodies - **First-line screening test** for all Rh-negative women at booking and again at 28 weeks - If negative: mother is unsensitised; proceed with anti-D prophylaxis - If positive: mother is sensitised; quantify antibody titre and plan further management (amniocentesis, cordocentesis, in-utero transfusion) **Clinical Pearl:** The titre of anti-D antibodies correlates with severity of haemolytic disease. A titre ≥1:16 (or >15 IU/mL) warrants further investigation with amniocentesis or cordocentesis. ### Why This Is the Investigation of Choice 1. **Non-invasive**: Requires only maternal blood sample 2. **Screening tool**: Identifies sensitisation status early in pregnancy 3. **Quantifiable**: Antibody titre guides management escalation 4. **Guideline standard**: Recommended by RCOG, ACOG, and Indian obstetric guidelines at booking and 28 weeks ### Comparison with Other Tests | Investigation | When Used | What It Detects | Role in Rh Disease | |---|---|---|---| | **Indirect Coombs** | Booking, 28 weeks | Free IgG anti-D in maternal serum | **Screening for sensitisation** | | **Direct Coombs** | After delivery, in neonate | Antibodies bound to fetal RBCs | Confirms haemolytic disease in baby | | **Kleihauer-Betke** | After delivery or sensitising event | Fetal cells in maternal circulation | Quantifies FMH to calculate anti-D dose | | **Amniocentesis + spectrophotometry** | If IAT positive + high titre | Bilirubin in amniotic fluid | Assesses severity in sensitised pregnancy | **Mnemonic:** **I-D-K-A** = **Indirect** (screening) → **Direct** (neonatal diagnosis) → **Kleihauer** (quantify bleed) → **Amniocentesis** (severity in sensitised). ### Management Algorithm ```mermaid flowchart TD A[Rh-negative woman at booking]:::outcome --> B[Indirect Coombs test]:::action B --> C{IAT positive?}:::decision C -->|No| D[Unsensitised]:::outcome C -->|Yes| E[Sensitised]:::outcome D --> F[Anti-D prophylaxis at 28 weeks + delivery]:::action E --> G[Quantify antibody titre]:::action G --> H{Titre ≥1:16?}:::decision H -->|No| I[Monitor IAT at 28 weeks]:::action H -->|Yes| J[Amniocentesis ± cordocentesis]:::action J --> K[Assess severity, plan in-utero transfusion if needed]:::action ```