A 28-year-old primigravida, Rh-negative, non-sensitised woman presents at 34 weeks of gestation with a history of antepartum haemorrhage (APH) of approximately 200 mL. She has no other complications. On examination, the uterus is tender and there is vaginal bleeding. Her indirect Coombs test is negative. What is the most appropriate immediate management?

Explanation

## Clinical Context This is an Rh-negative, non-sensitised woman with antepartum haemorrhage at 34 weeks. The key management principle is to prevent Rh sensitisation by administering anti-D immunoglobulin, but the dose depends on the volume of fetal red cells that have entered maternal circulation. ## Kleihauer–Betke Test: Why It Is Essential **Key Point:** The Kleihauer–Betke test quantifies the number of fetal red cells in maternal blood and allows calculation of the exact anti-D dose required. Standard dosing (500 IU/kg) may be insufficient if there is a large fetomaternal haemorrhage (FMH). **High-Yield:** In APH, FMH can exceed 4 mL of fetal red cells (8 mL of fetal blood), requiring additional anti-D beyond the standard 500 IU dose. The Kleihauer–Betke test detects this and guides supplemental dosing. ## Management Algorithm ```mermaid flowchart TD A[Rh-negative, non-sensitised woman with APH]:::outcome --> B{Quantify FMH}:::decision B -->|Kleihauer–Betke test| C[Calculate fetal RBC volume] C --> D{FMH > 4 mL RBC?}:::decision D -->|No| E[Standard anti-D 500 IU/kg]:::action D -->|Yes| F[Additional anti-D: 125 IU per mL fetal RBC]:::action E --> G[Recheck indirect Coombs at 72 hrs]:::action F --> G ``` ## Why Standard Dosing Alone Is Insufficient **Clinical Pearl:** A standard dose of anti-D (500 IU/kg, typically 100–500 IU) covers up to 4 mL of fetal red cells. If FMH exceeds this, the uncoated fetal red cells will sensitise the mother. The Kleihauer–Betke test prevents this by quantifying the exact requirement. **Mnemonic:** **FMH-KB** — Fetomaternal Haemorrhage requires Kleihauer–Betke quantification before anti-D dosing. ## Timing **High-Yield:** Anti-D must be given within 72 hours of the sensitising event (APH, amniocentesis, external cephalic version, etc.). Earlier administration is more effective. ## Why Indirect Coombs Negativity Does Not Negate Anti-D **Warning:** A negative indirect Coombs test means the mother is not yet sensitised, but APH is a sensitising event. Anti-D is given *prophylactically* to *prevent* sensitisation, not to treat existing sensitisation.