Rh(D) alloimmunisation and anti-D prophylaxis — structured oral station (12 minutes)

Opening prompt — "She has anti-D at 1:32 at booking. What does that mean and what do you do?"

Model response — say it in this order: ^[2]

• "This means she is already alloimmunised — she has formed anti-D antibodies, most likely from her first RhD-positive pregnancy, possibly despite anti-D, or from a missed sensitising event."
• "Anti-D prophylaxis is now pointless — anti-D is prevention, not treatment. I will not give her routine antenatal anti-D."
• "My job now is to detect fetal anaemia non-invasively and transfuse before hydrops."^[2]

Examiner is listening for: the prevention-versus-treatment distinction, and the shift to surveillance. ^[2]

Probe 1 — "Her first baby was healthy. Why is she sensitised now?"

• "Sensitisation can occur after a subclinical antenatal fetomaternal bleed before 28 weeks, a missed or late postpartum dose, or a sensitising event that was not covered. Most women are sensitised by small antenatal bleeds the programme cannot fully prevent."
• "I would take a focused history: any antepartum bleeding, procedures, or trauma in the first pregnancy, and confirm the postpartum anti-D was given within 72 hours."^[2]

Probe 2 — "How will you monitor this fetus?"

• "Because she had a previous healthy baby, this is a FIRST sensitised pregnancy — but the titre is already 1:32, which is at or above the critical titre of 1:16 by IAT. I will begin serial middle cerebral artery peak systolic velocity Doppler, every 1 to 2 weeks from 18 weeks."
• "Correct technique: proximal third of the MCA near its origin, near-field vessel, angle of insonation approaching zero degrees with no angle correction, and avoid head compression."
• "The trigger for fetal blood sampling with intrauterine transfusion readiness is an MCA-PSV above 1.5 multiples of the median for gestation."^[1][3]

Probe 3 — "If she had had a PREVIOUS baby who needed an intrauterine transfusion, would your monitoring change?"

• "Yes. In a previously affected pregnancy, titres are non-predictive because of the anamnestic response. I would skip titre monitoring and go straight to serial MCA-PSV from 16 to 18 weeks, because disease can declare early."
• "I would also refer her at the outset to a fetal medicine centre with intrauterine transfusion capability."^[2]

Probe 4 — "MCA-PSV rises above 1.5 multiples of the median at 24 weeks. What now?"

• "Refer for fetal blood sampling with intrauterine transfusion ready. At 24 weeks, delivery is riskier than the procedure, so we transfuse."
• "Technique: intravascular via cordocentesis into the umbilical vein at the placental insertion; O-negative, CMV-negative, irradiated, leucodepleted, crossmatch-compatible packed cells; volume to a post-transfusion haemoglobin around 14 to 15 g/dL."
• "Repeat guided by the predicted decline of about 1 g/dL per week and serial MCA-PSV, typically every 2 to 4 weeks until delivery."
• "Perinatal survival after intrauterine transfusion is over 90 percent; hydrops at the first transfusion reduces viable outcome by up to 11 percent, which is why we transfuse before hydrops."^[2][3]

Probe 5 — "How would you have PREVENTED this in the first place? Give me the schedule."

• "Routine antenatal prophylaxis: 625 IU IM at 28 weeks and 34 weeks. Postpartum: 625 IU IM within 72 hours if the infant is RhD-positive."
• "Sensitising events: 250 IU for events under 12 weeks, 625 IU for events over 12 weeks — covering antepartum haemorrhage, miscarriage over 12 weeks, ectopic, termination, trauma, amniocentesis, and external cephalic version."
• "I would not wait for the Kleihauer before giving the dose; the 625 IU covers 6 mL of fetal red cells, and additional anti-D at 100 IU per mL over 6 mL is given for a larger bleed."^[4]