A 32-year-old Rh-negative woman, G3P2, presents at 28 weeks gestation. Her first child (Rh-positive) had mild neonatal jaundice; her second child (Rh-positive) required phototherapy and exchange transfusion. Which single finding best distinguishes the severity of Rh isoimmunisation from the severity of ABO incompatibility in predicting fetal outcome?