An Rh-negative mother delivers an Rh-positive infant. The Kleihauer-Betke test shows 1.2% fetal cells in the maternal circulation. The dose of anti-D immunoglobulin required is:

• A Calculated dose: approximately 900 mcg (4500 IU) ✓
• B 300 mcg (1500 IU) — standard single dose
• C 600 mcg (3000 IU) — double standard dose
• D No additional dose needed if 300 mcg given within 72 hours

Explanation

Kleihauer-Betke test quantifies fetal-maternal haemorrhage (FMH). Each 1% of fetal cells represents approximately 50 mL of fetal blood (or 25 mL of fetal red cells) in maternal circulation. 1.2% = approximately 60 mL of fetal blood = 30 mL fetal RBCs. Each 300 mcg of anti-D covers 30 mL of fetal blood. So 1.2% fetal cells ÷ 0.4% covered per 300 mcg ≈ 3 vials = 900 mcg. A standard 300 mcg dose covers only up to 30 mL fetal blood (0.6% fetal cells); the excess FMH requires additional anti-D.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.