A 32-year-old Rh-negative woman, G3P2, presents at 28 weeks gestation. Her first child (Rh-positive) had mild neonatal jaundice; her second child (Rh-positive) required phototherapy and exchange transfusion. Which single finding best distinguishes the severity of Rh isoimmunisation from the severity of ABO incompatibility in predicting fetal outcome?

## Severity Discrimination: Antibody Titre Trend **Key Point:** In Rh isoimmunisation, **maternal anti-D antibody titre and its serial trend** are the most reliable predictors of fetal severity and distinguish it from ABO incompatibility, where titres are not clinically useful. ### Why Antibody Titre Matters in Rh Isoimmunisation **High-Yield:** The **critical titre** (usually 1:16 or 1:32, depending on laboratory) marks the threshold above which fetal hemolysis becomes significant. Serial titres across pregnancy show the anamnestic response and predict worsening hemolysis. ### Comparison Table: Antibody Titre in Rh vs ABO | Parameter | Rh Isoimmunisation | ABO Incompatibility | | --- | --- | --- | | **Antibody titre predictive value** | High — correlates with fetal severity | Low — titre does not predict severity | | **Critical titre** | 1:16–1:32 (lab-dependent) | Not applicable | | **Serial monitoring** | Essential for management | Not useful | | **Titre trend** | Rising titre → worsening prognosis | Titre remains stable or varies unpredictably | | **Clinical use** | Guides need for amniocentesis / Doppler | Not used for risk stratification | ### Clinical Application **Mnemonic:** **TITRE** = **T**racking **I**mmunisation **T**rend **R**equires **E**valuation 1. **Baseline titre at booking:** Establishes baseline risk. 2. **Repeat titres every 4 weeks:** If titre rises above critical level, proceed to amniocentesis or Doppler studies. 3. **Anamnestic response:** In subsequent pregnancies (as in this patient's history), titres rise faster and higher. ### Why Other Options Are Misleading **Clinical Pearl:** While anti-D antibodies *must* be present to diagnose Rh isoimmunisation, their mere presence does not predict severity — the **titre level and trend** do. Similarly, bilirubin in amniotic fluid is a sign of hemolysis but is detected *after* the decision to intervene has already been made based on titre. **Warning:** Do not confuse "presence of anti-D" with "anti-D titre." Presence is binary (yes/no); titre is quantitative and predictive. [cite:Park 26e Ch 8; RCOG Green-top Guideline 18]