An unsensitized Rh-negative woman received routine antenatal anti-D immunoglobulin at 28 weeks. After delivering an Rh-positive infant, a screening test for fetomaternal haemorrhage returns positive. What is the MOST likely explanation?

• A The antenatal anti-D has caused a true large fetomaternal bleed
• B The baby was actually Rh negative and mislabelled
• C The mother has developed immune anti-D despite prophylaxis
• D Passively administered anti-D gives a false-positive rosette test ✓

Explanation

The rosette test detects B-positive fetal cells using maternal serum anti-B. When the mother has recently received prophylactic anti-B, that passively acquired antibody coats fetal cells and produces a positive rosette even without significant fetomaternal haemorrhage. This is why a positive screen after antenatal anti-B must be quantified by Kleihauer-Betke or flow cytometry before deciding on additional dosing, rather than assumed to represent a large bleed.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.