In RhD isoimmunization, the Kleihauer-Betke (KB) test is performed to quantify fetomaternal haemorrhage. A woman has a KB test with 0.8% fetal cells in maternal blood. The mother's blood volume is estimated at 5000 mL. How many vials of anti-D immunoglobulin (300 µg/vial) would be required, given that each 300 µg anti-D protects against 15 mL of fetal red blood cells (or 30 mL whole fetal blood)?
- A 1 vial
- B 2 vials ✓
- C 3 vials
- D 4 vials
Correct answer: B. 2 vials
Explanation
Calculation: Fetal blood in maternal circulation = 0.8% × 5000 mL = 40 mL whole fetal blood (or 20 mL fetal RBCs). Each 300 µg vial covers 30 mL fetal whole blood (15 mL fetal RBCs). Number of vials = 40 mL ÷ 30 mL/vial = 1.33 vials → round up to 2 vials. An additional vial (rounding up to next whole number plus one extra per ACOG protocol) supports 2 vials. This calculation follows the standard formula: volume of FMH ÷ 30 mL per vial = number of 300 µg vials, always rounding up. One vial of 300 µg anti-D immunoglobulin neutralises approximately 30 mL of fetal whole blood.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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