A Rh-negative woman delivers a Rh-positive baby. The Kleihauer-Betke (acid-elution) test on maternal blood shows 0.6% fetal cells. The maternal blood volume is 5,000 mL. Calculate the volume of fetomaternal haemorrhage and the total dose of anti-D required (each 300 mcg vial protects against 30 mL fetal whole blood).

• A FMH = 30 mL; anti-D = 2 vials (600 mcg)
• B FMH = 50 mL; anti-D = 2 vials (600 mcg)
• C FMH = 30 mL; anti-D = 1 vial (300 mcg) ✓
• D FMH = 15 mL; anti-D = 1 vial (300 mcg)

Explanation

FMH volume = % fetal cells × maternal blood volume = 0.6% × 5,000 mL = 30 mL. Each 300 mcg vial of anti-D covers 30 mL of fetal whole blood (or 15 mL fetal red cells). Since FMH = 30 mL, one vial (300 mcg) is sufficient. An additional vial is given if FMH exceeds 30 mL; a 10% rounding-up margin is applied in practice. This calculation follows the standard British Committee for Standards in Haematology (BCSH) guideline.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.