A simplified method for large‐scale HPA‐la phenotyping for antenatal screening

Abstract

A simplified method for large-scale HPA-1a phenotyping of platelets was developed for use in an antenatal screening programme for fetomaternal alloimmune thrombocytopenia (FMAIT). The test was based on the MAIPA assay, which was modified for antigen-typing with a well-characterized anti-HPA-1a reagent. The resulting assay gave reliable results, was inexpensive and allowed testing of large batches using semiautomated equipment.

Antenatal Screening for Fetomaternal Alloimmune Thrombocytopenia: Should we be doing it?

“…A single pre‐delivery transfusion may protect against bleeding at the time of delivery, but the fetus remains at risk of spontaneous ICH earlier in pregnancy. Weekly in utero platelet transfusions have been shown to be effective in preventing ICH in severe cases of FMAIT (Murphy et al, 1994; Sainio et al, 1999), but this approach is invasive, as has already been discussed. Two recent studies support a less invasive approach with initial therapy with maternally administered IVIgG.…”

“…FBS for FMAIT should be performed at referral centres, where the risk of fetal loss is about 1% for each procedure (Murphy et al, 1994), and > 5% when serial transfusions are used (Murphy et al, 1998; Overton et al, 2002). The main problems are most likely with early FBS at around 20‐22 weeks’ gestation when the cord is small and the platelet count may be very low.…”

“…Typing for HPA‐1a seems to be the logical first step since 97.5% of women are HPA‐1a positive and can be excluded from further investigation. Reliable assays are available for HPA‐1a phenotyping using platelet‐rich plasma (Metcalfe et al, 1994), plasma without the need for platelet separation (Watkins et al, 1999), or whole blood (Bessos et al, 1996 & 1999; Watkins et al, 1999; Garner et al, 2000). Genotyping for HPA‐1a can be carried out, but it is not yet feasible on a large scale.…”

“…It is difficult to understand why there was such a difference in the success of maternal administration of IVIgG between these two studies. Relevant factors may include the methods used for assessing the success of treatment and the dose, timing and type of IVIgG used The selection of cases may also be important, and it has been suggested that therapeutic failures occur more frequently in severe cases (Murphy et al, 1994; Sainio et al, 1999). The direct injection of IgG to the fetus by cordocentesis was successful in raising the fetal platelet count in one case (Zimmerman & Huch, 1992).…”

Screening Programmes for Foetomaternal Alloimmune Thrombocytopenia

“…The choice is first between phenotyping to detect the HPA-la epitope itself, and genotyping, which detects the single base change giving rise to-the GP IIIa amino acid substitution which defines HPA-ldlb. A number of phenotyping assays using unstandardised platelet rich plasma [26] or whole blood [27] have been described. They generally involve some form of 'capture' monoclonal antibody to GPIIIa attached to a solid surface.…”