A Rh-negative primigravida at 28 weeks gestation has a Kleihauer-Betke test showing 0.8% fetal cells in maternal blood after an antepartum haemorrhage. The estimated fetomaternal haemorrhage (FMH) volume is:
- A 20 mL
- B 40 mL ✓
- C 80 mL
- D 10 mL
Correct answer: B. 40 mL
Explanation
The Kleihauer-Betke formula estimates FMH volume as: FMH (mL) = (percentage fetal cells / 100) × maternal blood volume. Assuming standard maternal blood volume of 5,000 mL: FMH = (0.8/100) × 5,000 = 40 mL. Each 300 mcg dose of anti-D immunoglobulin covers up to 15 mL of fetal red cells (30 mL whole blood). For 40 mL FMH: 40/15 ≈ 2.7, requiring 3 doses of 300 mcg anti-D, confirming that a standard single dose is insufficient.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.