An Rh-negative unsensitised woman delivers an Rh-positive baby. Kleihauer–Betke test shows a fetomaternal haemorrhage of 2 mL of fetal red blood cells. What is the most common reason for failure to prevent Rh isoimmunisation in this scenario?

Explanation

## Most Common Reason for Failure to Prevent Rh Isoimmunisation **Key Point:** Delayed administration of anti-D immunoglobulin beyond the critical 72-hour window is the most common reason for failure of prevention. The window of opportunity closes rapidly as fetal red blood cells are cleared from maternal circulation and maternal immune sensitisation occurs. ### The Critical 72-Hour Window **High-Yield:** Anti-D immunoglobulin must be administered **within 72 hours of delivery** to prevent sensitisation. This is the time frame during which fetal red blood cells are still present in maternal circulation and can be eliminated before triggering a primary immune response. ### Mechanism of Sensitisation Failure ```mermaid flowchart TD A[Delivery of Rh-positive baby]:::outcome --> B[Fetomaternal haemorrhage occurs]:::outcome B --> C{Anti-D given within 72 hours?}:::decision C -->|Yes| D[Fetal RBCs coated with IgG]:::action C -->|No| E[Maternal B cells encounter fetal RBCs]:::urgent D --> F[RBCs destroyed in spleen]:::action D --> G[Sensitisation prevented]:::outcome E --> H[Primary immune response initiated]:::urgent H --> I[Anti-D IgG produced over 6-12 weeks]:::urgent I --> J[Future pregnancies at risk of HDFN]:::urgent ``` ### Timeline of Sensitisation | Time Point | Event | Anti-D Efficacy | |-----------|-------|------------------| | 0–24 hours | FMH present, maternal B cells not yet activated | >95% effective | | 24–48 hours | Early immune response beginning | 90% effective | | 48–72 hours | Immune response progressing | 50–80% effective | | >72 hours | Sensitisation established, IgG production underway | <10% effective | **Clinical Pearl:** Even a 2 mL FMH (well within the standard 500 IU anti-D coverage) can cause sensitisation if anti-D is not given promptly. The dose is adequate, but timing is critical. ### Why This Is the Most Common Failure **Mnemonic: TIMING IS CRITICAL (TIC)** - **T**herapy must be given within 72 hours - **I**mmunoglobulin efficacy drops sharply after 72 hours - **C**ritical window closes as maternal immune response activates In clinical practice, delayed administration (due to administrative delays, lack of awareness, or delayed recognition of Rh-negative status) is the most frequent preventable cause of sensitisation. ### Optimal Management 1. **Identify Rh-negative unsensitised mothers** at delivery 2. **Confirm fetal/neonatal Rh status** (cord blood or baby's blood group) 3. **Quantify FMH** using Kleihauer–Betke or flow cytometry 4. **Calculate anti-D dose:** Standard 500 IU covers 4 mL fetal RBCs; additional doses for larger FMH 5. **Administer within 72 hours** (ideally within 24 hours) [cite:Williams Obstetrics 26e Ch 5; RCOG Green-top Guideline 22]