A Rh-negative woman delivers an Rh-positive neonate after a normal vaginal delivery. Kleihauer-Betke test shows 0.4% fetal cells in maternal circulation. To calculate the volume of fetal blood in maternal circulation: 0.4% × 50 = 200 mL fetal blood. The number of doses of anti-D immunoglobulin (300 µg per dose) required is:
- A One dose (300 µg)
- B Three doses (900 µg)
- C Two doses (600 µg) ✓
- D Four doses (1200 µg)
Correct answer: C. Two doses (600 µg)
Explanation
Each 300 µg dose of anti-D covers 30 mL of fetal whole blood (or 15 mL fetal RBCs). Fetal blood volume = 0.4% × 5000 mL maternal blood = 20 mL fetal RBCs = 40 mL fetal whole blood. This requires 40/30 = approximately 1.3 doses, rounded up to 2 doses (600 µg). One 300 µg standard dose covers up to 30 mL fetal blood; fetomaternal hemorrhage of 40 mL requires 2 doses. The formula: doses = (% fetal cells × 50) / 30, rounded up.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.