A Rh-negative mother delivers a Rh-positive baby. A Kleihauer-Betke test shows 2.2% fetal cells in maternal circulation. How many doses of anti-D immunoglobulin (300 mcg standard dose) are required to neutralise this fetomaternal hemorrhage?

• A 2 doses (600 mcg total), as each 300 mcg dose covers approximately 15 mL fetal red cells (30 mL whole blood)
• B 1 dose (300 mcg), as the standard dose is sufficient for FMH up to 30 mL whole blood
• C 3 doses (900 mcg total), to ensure complete protection with an additional safety margin
• D 4 doses (1200 mcg): 2.2% × 5000 mL ≈ 110 mL fetal whole blood; 110 ÷ 30 mL per 300 mcg dose = 3.67, rounded up to 4 ✓

Explanation

The Kleihauer-Betke test estimates the fetomaternal haemorrhage: volume of fetal whole blood ≈ (% fetal cells / 100) × maternal blood volume = (2.2/100) × 5000 ≈ 110 mL. Each 300 mcg dose of anti-D neutralises about 30 mL of fetal whole blood (15 mL of fetal red cells), so 110 ÷ 30 = 3.67, rounded up to 4 doses (1200 mcg); ACOG additionally recommends adding one further vial as a safety margin. A single dose or 2 doses covers only 30–60 mL and would be grossly inadequate for a 110 mL bleed, leaving the mother at risk of Rh alloimmunisation.

Reference: Williams Obstetrics, 26th ed.

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