What is the recommended dose of anti-D (Rho(D) immune globulin) prophylaxis?

Anti-D Prophylaxis Dosing Guidelines

The recommended dose of anti-D prophylaxis varies by clinical scenario, with 50 mcg (250 IU) for early pregnancy events (<12 weeks), 300 mcg (1500 IU) for postpartum prophylaxis, and 300 mcg at 28 weeks for antenatal prophylaxis.

First Trimester Events (<12-13 weeks)

• Early pregnancy loss/abortion (<12 weeks): 50 mcg (250 IU) dose is recommended 1, 2, 3, 4
• Administration should occur within 72 hours of the event, though it may still be effective if given up to 28 days after the event 2
• If the standard 50 mcg dose is unavailable, a 300 mcg dose can be substituted 1

Second/Third Trimester Events

• Events after 12-13 weeks: 300 mcg (1500 IU) dose is recommended 5, 2
• Specific indications requiring 300 mcg dose:
  • Amniocentesis
  • Cordocentesis
  • Abdominal trauma
  • Obstetric hemorrhage
  • Spontaneous or induced abortion after 12 weeks
  • Intrauterine fetal death

Standard Prophylaxis Protocol

• Antenatal prophylaxis: 300 mcg at 28 weeks' gestation 5, 2
• Postpartum prophylaxis: 300 mcg within 72 hours of delivery of an Rh-positive infant 5, 2
• If delivery occurs within 3 weeks after the last antenatal dose, the postpartum dose may be withheld unless there is significant fetomaternal hemorrhage 5

Large Fetomaternal Hemorrhage

• Standard 300 mcg dose covers up to 15 mL of fetal red blood cells (approximately 30 mL of fetal whole blood) 5
• For larger fetomaternal hemorrhage, additional dosing is required:
  • Calculate volume of fetal RBCs that entered maternal circulation
  • Administer 10 mcg additional anti-D for every 0.5 mL of fetal RBCs 2, 4
  • Formula: Number of doses = (Volume of fetal RBCs in mL) ÷ 15 mL 5
  • If calculation results in a fraction, round up to next whole number of doses 5

Route of Administration

• Anti-D can be administered either intramuscularly (IM) or intravenously (IV) with equal effectiveness 6
• For patients with bleeding disorders, use a fine needle (23 gauge or smaller) for IM administration 7
• Some products are only approved for IM administration and should never be given IV 5

Special Considerations

• In supply shortages, prioritize postpartum patients and those at later gestational ages 1, 7
• Documentation requirements include: date of administration, manufacturer, lot number, name and title of administrator 7
• If anti-D is not given within 72 hours, it should still be administered as soon as the need is recognized, for up to 28 days after the event 2

Common Pitfalls

1. Underdosing: Using 50 mcg dose for events after 12 weeks when 300 mcg is required
2. Timing errors: Delaying administration beyond 72 hours reduces effectiveness
3. Failure to calculate additional doses: Not accounting for large fetomaternal hemorrhage
4. Documentation lapses: Not properly recording administration details
5. Route errors: Administering IV when product is only approved for IM use

The Society for Maternal-Fetal Medicine (2024) emphasizes that while the evidence for first-trimester anti-D administration is not conclusive, the potential impact of alloimmunization on future pregnancies and the low risk of anti-D administration supports its use when logistically and financially feasible 1.