An Rh-negative primigravida delivers an Rh-positive baby. The Kleihauer-Betke test shows 1.5% fetal cells in maternal blood. What is the minimum dose of anti-D immunoglobulin required to cover this fetomaternal haemorrhage (FMH)?

• A 300 mcg (1500 IU)
• B 600 mcg (3000 IU)
• C 1200 mcg (6000 IU)
• D 900 mcg (4500 IU) ✓

Explanation

Using the BCSH/NICE calculation: FMH volume (mL whole blood) = KB% × total maternal blood volume (assumed 5000 mL) = 1.5% × 5000 = 75 mL fetal whole blood. Each 300 mcg anti-D covers approximately 30 mL fetal whole blood. Number of vials = 75 ÷ 30 = 2.5, rounded up to 3. Total anti-D = 3 × 300 mcg = 900 mcg (4500 IU). This is the minimum calculated dose; clinical practice rounds up to the next full vial.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.