A Rh-negative woman has an antepartum haemorrhage at 32 weeks. Kleihauer-Betke test shows 1.2% fetal cells in maternal circulation. Using the standard formula (maternal blood volume = 5000 mL), the estimated fetomaternal haemorrhage (FMH) and the required anti-D dose (300 mcg per 30 mL fetal blood) are:
- A FMH = 60 mL; anti-D = 300 mcg
- B FMH = 60 mL; anti-D = 600 mcg ✓
- C FMH = 30 mL; anti-D = 300 mcg
- D FMH = 120 mL; anti-D = 1200 mcg
Correct answer: B. FMH = 60 mL; anti-D = 600 mcg
Explanation
FMH (mL) = % fetal cells × maternal blood volume = 1.2/100 × 5000 = 60 mL of fetal blood. Anti-D required = 60 mL ÷ 30 mL per dose × 300 mcg = 2 × 300 = 600 mcg. One standard 300 mcg vial covers 30 mL fetal RBCs, so 60 mL requires two vials (600 mcg total). Always round up to the next full vial.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.