A Rh-negative woman with anti-D titre of 1:64 at 24 weeks is being monitored. Which investigation best predicts fetal anaemia requiring intervention at this gestation?

• A Amniotic fluid optical density at 450 nm (delta OD 450) on amniocentesis
• B Serial maternal anti-D titre rising above 1:256
• C Fetal scalp blood sampling for haemoglobin
• D Middle cerebral artery peak systolic velocity (MCA-PSV) >1.5 MoM on Doppler ultrasonography ✓

Explanation

MCA-PSV Doppler measurement is the current gold standard non-invasive test for predicting fetal anaemia in Rh isoimmunisation. In anaemic fetuses, increased cardiac output and decreased blood viscosity accelerate flow velocity in the middle cerebral artery; MCA-PSV >1.5 multiples of the median (MoM) for gestation has 100% sensitivity for moderate-to-severe anaemia requiring intrauterine transfusion (IUT). Delta OD 450 (Liley chart) is now largely superseded by MCA-PSV. Anti-D titre monitoring has poor predictive value at high levels. Fetal scalp sampling is only possible during labour.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.