A Rh-negative unsensitised woman delivers a Rh-positive infant. The Kleihauer-Betke test on maternal blood reveals that 0.8% of her red blood cells are fetal. The calculated fetal-maternal haemorrhage volume is approximately 40 mL of fetal whole blood. How many standard 300 μg doses of anti-D immunoglobulin should she receive?

• A Two doses (600 μg total) — one standard + one additional for excess FMH ✓
• B One dose (300 μg) — standard postpartum dose
• C Three doses (900 μg total)
• D One dose is adequate as Kleihauer-Betke confirms sensitisation has already occurred

Explanation

One standard 300 μg dose of anti-D immunoglobulin covers approximately 15 mL of fetal red blood cells (30 mL of fetal whole blood). For a fetal-maternal haemorrhage of 40 mL fetal whole blood, the standard single dose is insufficient. One additional 300 μg dose is needed to cover the excess 10 mL (40 mL − 30 mL), giving a total of two doses (600 μg). Each additional 300 μg covers a further 30 mL fetal whole blood or 15 mL fetal RBCs.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.