A Rh-negative woman delivers an Rh-positive infant. The Kleihauer-Betke (acid elution) test shows 2.5% fetal cells in maternal blood. What volume of fetal haemorrhage does this represent and what dose of Rh immunoglobulin (RhIg) is required?

• A ~50 mL fetomaternal haemorrhage; 1 vial (300 µg) RhIg IM
• B ~125 mL fetomaternal haemorrhage; 1 vial (300 µg) RhIg IM
• C ~50 mL fetomaternal haemorrhage; 2 vials (600 µg total) RhIg IM
• D ~125 mL fetomaternal haemorrhage; 5 vials (1500 µg total) RhIg IM ✓

Explanation

The Kleihauer-Betke test estimates the fetomaternal haemorrhage: volume of fetal whole blood = (% fetal cells / 100) × maternal blood volume ≈ (2.5/100) × 5000 = 125 mL. Each 300 µg vial of anti-D neutralizes about 30 mL of fetal whole blood (15 mL of packed fetal red cells), so 125 ÷ 30 = 4.17, rounded up to 5 vials (1500 µg); ACOG additionally recommends adding one vial as a safety margin. A single 300 µg vial covers only ~30 mL of fetal blood and is grossly inadequate for a 125 mL bleed, so the options giving 1–2 vials would leave the mother under-dosed and at risk of Rh alloimmunization.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.