Overestimation of fetomaternal haemorrhage by the acid‐elution technique in mothers with β‐thalassaemia minor

Summary. In Rh‐negative women, it is important to quantify the magnitude of an Rh‐positive fetomaternal haemorrhage (FMH) so that sufficient Rh immune globulin (RhIg) can be administered early in the postpartum period to prevent alloimmunization. The standard post‐partum dose of Rhlg varies from 100 μg in the UK to 300 μg in North America. It is therefore important to identify all Rh‐negative women who have had an FMH greater than 10 ml in the UK or greater than 30 ml in North America because an FMH greater than these amounts will affect the dose of RhIg that is administered. As acid‐elution techniques can overestimate the magnitude of an FMH in the presence of an elevated maternal haemoglobin F level, we performed a prospective study to determine how often this occurred. Of 1,894 consecutive Rh‐negative mothers who delivered Rh‐positive infants, whose blood was screened for an FMH greater than 10 ml of fetal blood using an acid‐elution procedure, 11 were found to have an FMH over 10 ml. In five of these 11 women, the volume of FMH was less than 10 ml using an alternative technique (rosette test) to assess the FMH size. Six of these women were found to have β‐thalassaemia minor on the basis of a low MCV, and high haemoglobin A 2 and/or high haemoglobin F levels. In five of these the FMH was significantly overestimated by the acid‐elution technique compared to the rosette technique. Therefore, in the presence of a maternal condition, which may result in an elevated haemoglobin F level, an FMH estimated to be over 10 ml in the UK or 30 ml in North America using an acid‐elution procedure, should be confirmed by an alternative technique, which does not involve the estimation, directly or indirectly, of haemoglobin F.