An Rh-negative primigravida delivers a 3.4 kg Rh-positive baby. A Kleihauer-Betke test is performed on maternal blood. The result shows 0.5% fetal cells in maternal blood. The volume of feto-maternal haemorrhage (FMH) and the additional dose of anti-D immunoglobulin required (above the routine 300 µg) are calculated. The estimated volume of FMH is:
- A 15 mL
- B 25 mL ✓
- C 40 mL
- D 50 mL
Correct answer: B. 25 mL
Explanation
FMH volume = % fetal cells × 50 (maternal blood volume ~5000 mL assumed). FMH = 0.5% × 50 = 25 mL of fetal whole blood. The standard Kleihauer-Betke formula: volume of FMH (mL) = (% fetal cells / 100) × maternal blood volume (5000 mL). 0.5/100 × 5000 = 25 mL. Each 300 µg anti-D covers 30 mL fetal whole blood; 25 mL is covered by the standard 300 µg dose. No additional anti-D is required beyond the routine dose in this case.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.