The Kleihauer-Betke test in a Rh-negative mother after delivery detects fetal cells occupying 0.5% of the maternal red cell population. The maternal blood volume is approximately 5000 mL with a haematocrit of 36%. How many vials of anti-D immunoglobulin (standard dose 300 mcg each) are required to cover this fetomaternal haemorrhage?
- A 1 vial
- B 2 vials ✓
- C 3 vials
- D 4 vials
Correct answer: B. 2 vials
Explanation
The calculation: Fetal red cell volume = 0.5% × 5000 mL × 0.36 = 9 mL of fetal red cells. Each vial of 300 mcg anti-D covers approximately 15 mL of fetal red cells (300 mcg neutralises 30 mL of whole fetal blood = ~15 mL RBCs). Therefore, 9 mL ÷ 15 mL/vial = 0.6 vials → rounded up to 1 full vial. However, standard practice adds one extra vial as a margin of safety, giving 2 vials total. ACOG recommends rounding up to the next whole vial and adding one additional for safety when the calculated dose is >1 standard vial.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.