A Rh-negative woman delivers a Rh-positive baby. The Kleihauer-Betke test shows a fetomaternal hemorrhage (FMH) of 35 mL of fetal blood. Each vial of anti-D immunoglobulin (300 mcg/1500 IU) covers a FMH of 30 mL fetal RBCs. How many vials are required?
- A 1 vial
- B 2 vials ✓
- C 3 vials
- D 4 vials
Correct answer: B. 2 vials
Explanation
Each 300 mcg vial of anti-D neutralizes 30 mL of fetal whole blood (or 15 mL of fetal packed red cells). For 35 mL of fetal whole blood: 35/30 = 1.17 vials, rounded up to 2 vials. The standard rule is to round up to the next whole number and add 1 vial for rounding — though strictly 35/30 rounds to 2. Most guidelines state: divide FMH volume by 30, round up to the nearest whole number; for 35 mL: 1.17 → 2 vials. This is the correct calculation.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.