A Rh-negative woman delivers a Rh-positive baby at term. A Kleihauer-Betke (KB) test is performed and reveals 1.2% fetal cells. Using the KB test to calculate the volume of fetomaternal hemorrhage (FMH) and the anti-D dose required (using the formula: FMH mL = KB% × maternal blood volume / 100, maternal blood volume = 5000 mL; 1 vial anti-D = 300 µg neutralises 15 mL fetal RBCs), the required anti-D dose is:

• A 300 µg (1 vial)
• B 900 µg (3 vials)
• C 1200 µg (4 vials)
• D 600 µg (2 vials) ✓

Explanation

FMH volume = (KB% / 100) × maternal blood volume = (1.2/100) × 5000 = 60 mL whole blood = 30 mL fetal RBCs (whole blood is approximately 50% RBCs). Each 300 µg vial covers 15 mL fetal RBCs: 30/15 = 2 vials = 600 µg. An additional vial is given when FMH exceeds a threshold to cover rounding. Therefore, 600 µg (2 vials) is the correct dose. If FMH were >30 mL fetal cells, 3 vials would be needed; an extra vial is added if the calculation is not a whole number.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.