Fetal and Neonatal Alloimmune Thrombocytopenia-New Prospects for Fetal Risk Assessment of HPA-1a-Negative Pregnant Women.

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare and potentially serious bleeding condition in the fetus/newborn. FNAIT is usually considered as the platelet counterpart of hemolytic disease of the fetus and newborn. In FNAIT, maternal alloantibodies against paternally inherited platelet antigens traverse the placenta and cause thrombocytopenia in the fetus/newborn. The most common and most serious cases of FNAIT among white people are caused by alloantibodies against the human platelet antigen 1a (HPA-1a), which is absent in 2.3% of women. Today, there is no screening for FNAIT, and for this reason, FNAIT is not suspected until an otherwise healthy child, born at term, presents with thrombocytopenia. Clinical management of subsequent pregnancies at risk of FNAIT is mostly based on the obstetric history. During the last 5 decades, hemolytic disease of the fetus and newborn caused by antibodies against RhD has successfully been prevented by administration of hyperimmune anti-D IgG drug products to RhD-negative women after delivery of an RhD-positive child. Similarly, a hyperimmune anti-HPA-1a IgG (NAITgam) is under development for the prevention of HPA-1a immunization and FNAIT. If NAITgam becomes licensed for FNAIT prophylaxis and national health authorities decide to include FNAIT screening in their antenatal health care programs, it will be necessary to improve today's tools for assessing the risk of FNAIT. Although the primary risk factor for HPA-1a immunization is platelet type HPA-1bb, not all HPA-1a-negative women develop anti-HPA-1a. The women who are HLA-DRB3:01:01 negative (72%) only rarely develop anti-HPA-1a, and for those few who become HPA-1a immunized, it is quite rare to have a child with severe thrombocytopenia. Determination of fetal HPA-1 type is important because 15% of HPA-1a-negative women will carry an HPA-1a-negative fetus and therefore not be at risk of FNAIT. The severity of FNAIT seems to be associated with the level of anti-HPA-1a. Hence, in Norway, for example, an Ab threshold of 3 IU/mL is used to distinguish between low- and high-risk pregnancies. The current review will discuss to what extent these analyses, as well as determination of subtypes of anti-HPA-1a (anti-ß3, anti-αIIbß3, and anti-αvß3) and Fc core fucosylation of anti-HPA-1a IgG, can be used as risk stratification tools.

Non-invasive risk-assessment and bleeding prophylaxis with IVIG in pregnant women with a history of fetal and neonatal alloimmune thrombocytopenia: management to minimize adverse events.

INTRODUCTION: In pregnant women with a history of fetal and neonatal alloimmune thrombocytopenia (FNAIT), prenatal intervention in subsequent pregnancies may be required to prevent fetal bleeding. Several invasive and non-invasive protocols have been published: amniocentesis for fetal genotyping, fetal blood sampling for the determination of fetal platelet count, intrauterine platelet transfusions, and weekly maternal i.v. immunoglobulin (IVIG) infusion with or without additional corticosteroid therapy. This is the first retrospective study that report the experience with a non-invasive protocol focused on side effects of maternal IVIG treatment and neonatal outcome. METHODS: Pregnant women with proven FNAIT in history and an antigen positive fetus were treated with IVIG (1 g/kg/bw) every week. To identify potential IVIG-related hemolytic reactions isoagglutinin titer of each IVIG lot and maternal blood count were controlled. IVIG-related side effects were prospectively documented and evaluated. Furthermore, ultrasound examination of the fetus was performed before starting IVIG administration and continued regularly during treatment. Outcome of the index and subsequent pregnancy was compared. Corresponding data of the newborns were analyzed simultaneously. RESULTS: IVIG was started at 20 weeks of gestation (median). Compared to the index pregnancy, platelet counts of the newborns were higher in all cases. No intracranial hemorrhage occurred (Index pregnancies: 1 case). Platelet counts were 187 × 109/l (median, range 22-239, 95% CI) and one newborn had mild bleeding. No severe hemolytic reaction was observed and side effects were moderate. CONCLUSION: Among pregnant women with FNAIT history, the use of non-invasive fetal risk determination and maternal IVIG resulted in favorite outcome of all newborns. Invasive diagnostic or therapeutic procedures in women with a history of FNAIT should be abandoned.

Fast and low-cost direct ELISA for high-throughput serological HPA-1a typing.

BACKGROUND: Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by maternal alloantibodies against fetal human platelet antigens (HPAs), mostly caused by anti-HPA-1a. Population-based screening for FNAIT is still a topic of debate. Logistically and financially, the major challenge for implementation is the typing of pregnant women to recognize the 2% HPA-1a-negative women. Therefore, there is need for a high-throughput and low-cost HPA-1a-typing assay. STUDY DESIGN AND METHODS: A sandwich ELISA was developed, using a monoclonal anti-GPIIIa as coating antibody and horseradish-peroxidase-conjugated recombinant anti-HPA-1a, as detecting antibody. The ELISA results were compared to an allelic discrimination PCR-assay. In phase I, samples from unselected consecutive pregnant women were tested with both assays. Phase II was part of a prospective screening study in pregnancy and genotyping was restricted to samples with an arbitrary set, OD < 0.500. RESULTS: The ELISA was optimized to require no additional handling (swirling or spinning) of stored tubes. During phase I, 506 samples were tested. In phase II, another 62,171 consecutive samples were phenotyped, with supportive genotyping in 1,902. In total 1,585 HPA-1a negative and 823 HPA-1a positive women were genotyped. The assay reached 100% sensitivity with a cut-off OD from 0.160, corresponding with a 99.9% specificity and a false-HPA-1a negative rate of 0.03. CONCLUSION: A high-throughput, low-cost, and reliable HPA-1a phenotyping assay was developed which can be used in population-based screening to select samples for testing of presence of anti-HPA-1a. Because plasma from tubes of 3- to 6-days-old samples can be used, this assay is applicable to settings with suboptimal conditions.

How do we treat fetal and neonatal alloimmune thrombocytopenia?

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a potentially devastating disease, seen in one in 800 to 1000 neonates. FNAIT is the most common cause of early-onset isolated severe neonatal thrombocytopenia in maternity wards. The complication of this disorder most to be feared is intracranial hemorrhage, leading to death or to neurologic sequels. As there is no systematic screening of at-risk pregnancies, FNAIT is often discovered when signs of bleeding are observed during pregnancy or at delivery. Platelet transfusion is required in case of bleeding or severe thrombocytopenia (<30 × 10(9) /L) during the 48-hour-postdelivery period. Diagnosis of alloimmunization is important for management of the index case and for subsequent pregnancies, due to the increasing severity of this syndrome as it recurs. Noninvasive antenatal therapy is based on maternal perfusion of intravenous immunoglobulins and risk stratification. In our experience, the addition of corticoids during the last trimester significantly improves the efficiency of treatment. Follow-up of antibody concentration during pregnancy may constitute a useful variable for therapy effectiveness.