A Rh-negative woman delivers a Rh-positive baby. Kleihauer-Betke test identifies 4 mL of fetal red cells in the maternal circulation. Given that 500 IU of anti-D covers 4 mL of fetal red cells, the required dose of anti-D immunoglobulin is:
- A 250 IU
- B 500 IU ✓
- C 1000 IU
- D 1500 IU
Correct answer: B. 500 IU
Explanation
The standard calculation: 500 IU of anti-D covers 4 mL of fetal red cells. For a fetomaternal haemorrhage of exactly 4 mL fetal cells, 500 IU is the appropriate dose. This should be given within 72 hours of delivery. Doses are rounded up to the nearest available vial size. This calculation applies the principle of 125 IU per mL of fetal packed red cells.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.