Fetal transfusion of red blood cells for alloimmunization: validity of a published equation

To validate the equation published in 1990 by Leduc et al. for red blood cell fetal transfusion where fetoplacental blood volume (VO) = 100 ml/kg, then improve its precision.

We reviewed 101 fetal transfusions among 32 patients. We analyzed risk factors for an inaccurate estimation with uni- and multivariate analysis. We compared the obtained Leduc formula with three other published equations.

Fetal weight and gestational age were risk factors for an inaccurate estimation of the final Hct. Before 32 weeks the estimation of VO was 120 ml/kg instead of 100 ml/kg. All formulae overestimated the mean expected Hct value. However, expected Hct estimated by Leduc's formula is the nearest of the observed final Hct.

Leduc's equation seems to be accurate, but less so for the youngest fetuses. We propose an adapted formula VO according to gestational age and fetal weight estimation.