A Rh-negative unsensitized woman at 28 weeks gestation receives antenatal anti-D 300 mcg IM. She then has a vaginal delivery at 38 weeks. A Kleihauer-Betke (KB) test is performed on maternal blood within 1 hour of delivery, showing 1.2% fetal cells. How many additional vials of anti-D (300 mcg each) should she receive?
- A Two additional vials (600 mcg) — calculated for FMH of approximately 60 mL fetal whole blood ✓
- B One additional vial (total 300 mcg) — the 28-week dose should provide adequate coverage
- C Three additional vials (900 mcg) — for FMH >30 mL fetal red cells requiring enhanced prophylaxis
- D No additional anti-D — the antenatal prophylaxis dose at 28 weeks covers postpartum immunization risk
Explanation
The Kleihauer-Betke test quantifies fetal-maternal hemorrhage. The calculation: fetal cells (%) × maternal blood volume. Standard maternal blood volume is ~5,000 mL. Fetal whole blood volume FMH = 1.2/100 × 5000 = 60 mL fetal whole blood = 30 mL fetal red cells (packed). Each 300 mcg anti-D covers approximately 30 mL fetal whole blood (15 mL fetal RBCs). Therefore 60 mL FMH requires 2 × 300 mcg = 600 mcg anti-D. The standard routine postpartum dose (1 vial/300 mcg) covers up to 30 mL FMH. Since FMH is 60 mL, 2 additional vials (600 mcg) are required. The 28-week antenatal dose is NOT counted toward postpartum dosing — antenatal prophylaxis has typically been cleared from circulation by delivery, so full postpartum dosing is required based on KB result.
Reference: Williams Obstetrics, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.