A Rh-negative woman delivers a Rh-positive baby after an uncomplicated vaginal delivery. The Kleihauer-Betke (KB) test is performed and shows 0.8% fetal cells. The calculated feto-maternal haemorrhage (FMH) is approximately 40 mL of fetal blood. The dose of anti-D required is:

• A 300 µg (1500 IU) anti-D
• B 150 µg (750 IU) anti-D
• C 600 µg (3000 IU) anti-D ✓
• D 1200 µg (6000 IU) anti-D

Explanation

The standard anti-D dose of 300 µg covers FMH up to 30 mL of fetal whole blood (approximately 15 mL fetal cells). For FMH of 40 mL fetal blood, which exceeds the standard coverage threshold, an additional 300 µg is needed. Therefore total dose is 600 µg (3000 IU). The formula is: FMH (mL whole blood) / 30 mL per 300 µg dose, rounded up to the nearest vial. 40 mL / 30 mL = 1.33, rounded up to 2 vials = 2 × 300 µg = 600 µg.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.