A Rh-negative mother delivers a Rh-positive infant. Kleihauer-Betke test on maternal blood shows 0.6% fetal cells (maternal blood volume = 5000 mL). The estimated volume of fetomaternal hemorrhage (FMH) and the minimum dose of anti-D immunoglobulin required are:

• A FMH = 3 mL fetal whole blood; anti-D 75 mcg (0.25 vial) IM
• B FMH = 30 mL fetal whole blood; anti-D 300 mcg (1 vial) IM ✓
• C FMH = 60 mL fetal whole blood; anti-D 600 mcg (2 vials) IM
• D FMH = 15 mL fetal whole blood; standard 300 mcg dose covers this volume

Explanation

FMH (mL of fetal red cells) = (% fetal cells / 100) × maternal blood volume = (0.6/100) × 5000 = 30 mL fetal red cells. Since fetal blood has a higher haematocrit (~50%), 30 mL fetal red cells corresponds to approximately 30 mL fetal whole blood for dosing purposes. Each 300 mcg (1500 IU) vial of anti-D neutralises up to 30 mL of fetal whole blood (15 mL fetal packed RBCs). Therefore, 30 mL FMH requires exactly 1 vial (300 mcg) of anti-D immunoglobulin IM. Always round up and add one extra vial when the calculation falls at the threshold to ensure complete coverage.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.