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Blood, Vol. 92 No. 7 (October 1), 1998:
pp. 2280-2287
The Natural History of Fetomaternal Alloimmunization to the
Platelet-Specific Antigen HPA-1a (PlA1, Zwa) as
Determined by Antenatal Screening
By
Lorna M. Williamson,
Gerald Hackett,
Janet Rennie,
Christopher R. Palmer,
Caroline Maciver,
Ruth Hadfield,
Darren Hughes,
Shirley Jobson, and
Willem H. Ouwehand
From the Division of Transfusion Medicine, University of Cambridge,
Cambridge, UK; the National Blood Service, East Anglia and Birmingham
Centres, UK; the Departments of Obstetrics and Paediatrics,
Addenbrooke's Hospital, Cambridge, UK; Centre for Applied Medical
Statistics, Department of Community Medicine, University of Cambridge,
Cambridge, UK; and the National Institute for Biological Standards and
Controls, Potters Bar, UK.
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ABSTRACT |
Immunization against the human platelet antigen (HPA)-1 alloantigen
is the most common cause of severe fetal and neonatal thrombocytopenia.
Fetal therapy has substantial risks and its indications need better
definition. Of 24,417 consecutive pregnant women, 618 (2.5%) were
HPA-1a negative of whom 385 entered an observational study. All were
HLA-DRB3*0101 genotyped and screened for anti-HPA-1a. Their partners
and neonates were HPA-1 genotyped and the latter were assessed by cord
blood platelet counts and cerebral ultrasound scans. Anti-HPA-1a was
detected in 46 of 387 pregnancies (12.0%; 95% CI 8.7%-15.2%). All
but one were HLA-DRB3*0101 positive (odds ratio 140; 95% CI 19-1035;
P< .00001). One baby died in utero, and of 26 HPA-1a-positive babies born to women with persistent antenatal
antibodies, 9 were severely thrombocytopenic (8 with a count <10 × 109/L, 1 with a large porencephalic cyst), 10 were mildly
thrombocytopenic, whereas 7 had normal platelet counts. Severe
thrombocytopenia was significantly associated with a third trimester
anti-HPA-1a titer  1:32 (P = .004), but was not observed
in babies of women with either transient or postnatal-only antibodies.
HPA-1a alloimmunization complicates 1 in 350 unselected pregnancies,
resulting in severe thrombocytopenia in 1:1,200. HPA-1a and
HLA-DRB3*0101 typing combined with anti-HPA-1a titration allows
selection of the majority of pregnancies at risk of severe
thrombocytopenia.
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INTRODUCTION |
FETOMATERNAL ALLOIMMUNIZATION to paternal
platelet-specific antigens on fetal platelets is the most common cause
of severe thrombocytopenia in neonates.1 Although this
condition is usually referred to as neonatal alloimmune
thrombocytopenia (NAITP), intrauterine death or intracerebral
hemorrhage can occur by 20 to 24 weeks of pregnancy.2 Most
cases show disparity in the biallelic human platelet antigen (HPA)-1
system,3 with an HPA-1a (previously, PlA1 or
Zwa) -negative woman and an HPA-1a-positive
fetus.4 Approximately 2% to 2.5% of the white population
is HPA-1a negative; however, the chance of HPA-1a alloimmunization is
strongly associated with maternal HLA class-II DRB3*0101 (DR52a)
type.5
Most severe cases of NAITP are diagnosed in the neonatal period, which
justifies fetal blood sampling and antenatal therapy in subsequent
pregnancies. Intravenous administration of high doses of immunoglobulin
G to the mother6 and intrauterine transfusions of
compatible platelets7 have been successfully used as
antenatal therapies; however, they are associated with the possibility
of nonresponse leading to ICH8 or fetal loss from
hemorrhage9, respectively. Because both
large-scale phenotyping suitable for pregnancy10 and
genotyping of fetuses for HPA-1a11 are technically
possible, some investigators have proposed antenatal screening for
HPA-1a alloimmunization with interventional therapy to prevent
ICH.12
Previous small-scale prospective studies have been
undertaken,13-16 but the wide range of alloimmunization
reported (0%-10%), combined with the small number of affected babies
studied, prohibits meta-analysis of the outcome of these studies. For
families with HPA-1a alloimmunization but with no previously affected
children, knowledge of the likely outcome is still limited. No
predictors of severe disease have been established that permit
selection of cases for fetal blood sampling with a view to antenatal
treatment. We therefore aimed to elucidate the natural history of
HPA-1a alloimmunization in an observational study that examined the
relationship between maternal antibodies to HPA-1a, cord platelet
count, and infant morbidity in a cohort of HPA-1a-negative women
identified by HPA-1a phenotyping of approximately 25,000 nonselected
pregnant women.
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METHODS |
Patient selection and management.
The initial study cohort was all pregnant women in the predominantly
(>95%) white East Anglian region of England, from whom blood samples
were received for testing in the regional program to prevent hemolytic
disease of the newborn. This program receives samples from greater than
98% of all pregnancies in the region. The study received approval
before commencement from the Ethics Committee of each of the nine
participating hospitals, and was also approved by Professional Regional
Advisory Committees in hematology, obstetrics, and pediatrics. A total
of 24,417 consecutive EDTA anticoagulated blood samples received
between September 1993 and September 1994 were typed for HPA-1a. Women
found to be HPA-1a negative were sent an information leaflet and
consent form. Subsequent investigations and management of
HPA-1a-negative women who consented to join the study are summarized
in Fig 1. No further
investigations were permitted in women who did not enroll and give
informed consent. A research midwife dedicated to the study
(C.M.) was available to answer queries from study
participants and hospital staff.
Women with detectable anti-HPA-1a were contacted for a full family
history, and received written advice on avoidance of aspirin and
vigorous exercise according to international guidelines.17 An individualized pregnancy care/delivery plan was decided by the local
obstetrician in consultation with the obstetrician in the study team
(G.H.), and depended on previous obstetric history. Invasive procedures such as amniocentesis for fetal HPA-1 typing and
fetal blood sampling were not part of the investigational protocol, but
were available if requested (by kind arrangement with Prof Charles
Rodeck, University College Hospital, London, UK).
Cord blood platelet counts were performed on the study cases and 200 normal babies as controls.
HPA-1a phenotyping assay.
The assay was a two-plate modification of antigen-capture design
enzyme-linked immunosorbent assay (ELISA) assay.18 Briefly, a microtiter plate (Maxisorp; Nunc, Roskilde, Denmark) was precoated with goat antibodies to mouse immunoglobulin G (Jackson Immunoresearch Inc, W Baltimore, PA), and after washing, incubated with a mouse monoclonal antibody (MoAb) to glycoprotein (GP) IIb/IIIa (CD 41) [CLB-C17 culture supernatant; provided by Prof A.E.G. Kr. von dem
Borne, Central Laboratory of the Netherlands Red Cross Blood Transfusion Service, Amsterdam, The Netherlands (CLB)], then washed again. Platelet-rich plasma from each sample to be typed (200 µL, not
adjusted for platelet count), was added to a second microtiter plate.
After centrifugation and washing, the platelet pellets were solubilized
in 200 µL of 1% Nonidet P-40 in Tris-buffered saline at
4°C for 1 hour, and 100 µL transferred to
corresponding wells in the initial plate. After incubation and washing,
100 µL of a 1:100 dilution of human plasma containing a high titer
IgG anti-HPA-1a but no reactivity against mouse immunoglobulin
was added. After incubation and washing, alkaline
phosphatase-conjugated goat antibodies to human immunoglobulin G
(Jackson Immunoresearch) was added to each well and incubated for 1 hour, followed by four washes. Color was developed by adding
p-nitrophenyl phosphate as substrate and absorbance was measured
(Titertek Multiscan, version 1.4; Quest Biomedical, Knowle,
UK). Each plate included a reagent blank and two wells
each of control platelets of the HPA-1a1a, 1a1b, and 1b1b type. Cut-off
values were established based on absorbance values obtained with the
positive and negative controls. Evaluations of HPA-1a typing after the
first 3 months showed that two HPA-1a-negative women had been typed
HPA-1a positive and the cut-off value was redefined for the final 9 months to reduce the risk of false-positive HPA-1a phenotypes.
All women who enrolled for the study, and whose initial typing result
was HPA-1a negative were retyped on a fresh EDTA anticoagulated sample
using a polyclonal IgG anti-HPA-1a in the platelet immunofluorescence test19 read by flow cytometry (Becton Dickinson FACScan;
Becton Dickinson UK Limited, Oxford, UK).
HPA-1 genotyping.
Genomic DNA from partners and babies of HPA-1a alloimmunized women was
used for HPA-1 genotyping by polymerase chain reaction amplification of
a 482-base-pair fragment of the GPIIIa gene, followed by restriction
with MsP1 and subsequent fragment length polymorphism analysis by gel
electrophoresis of the digested DNA.20 If cord blood
samples were not received, amplification was performed by a modified
method21 with samples of dried blood spots from cards taken
for phenylketonuria screening.
Detection and titration of HPA antibodies.
Samples from HPA-1a-negative women were screened for antibodies
against HPA-1a, HPA-3a, and HPA-3b by a standard MoAb immobilization of
platelet glycoprotein assay (MAIPA),22 using
the MoAb CLB-C17 to GP IIb/IIIa, and two reagent platelets homozygous
for for HPA-1 and -3 antigens. If antibodies were found, the timing of
immunization was assessed by retrospective testing of stored samples
previously sent for HPA/HLA typing. Titration of anti-HPA-1a was
performed in the MAIPA in duplicate, with twofold plasma dilutions in
buffered phosphate-buffered saline (PBS) from undiluted to 1 in 512.
In cases in which neonatal thrombocytopenia was present, and the mother
apparently negative for anti-HPA-1a, extra testing was performed for
HPA antibodies. Maternal blood was retested for anti-HPA-1a in the
MAIPA using a different antibody to capture GPIIIa (Y2.51, supplied by
Dr D. Mason, Oxford, UK), and also in a solid-phase assay according to
manufacturer's instructions (supplied by GTI, Brookfield, WI). Such
samples were also examined for the presence of antibodies against the
alloantigens of the HPA-2 and HPA-5 systems in the MAIPA assay using
the MoAbs CLB-MB45 against GP Ib/IX (CD42b) and CLB-10G11 against GP
Ia/IIa (CD49b) [both provided by Prof A.E.G. Kr. von dem Borne, CLB].
A four-cell platelet panel with homozygous expression of the HPA-1, -2, -3, and -5 alloantigens was used in these investigations.
HLA-DRB3*0101 genotyping.
The presence of the HLA-DRB3*0101 allele was determined by PCR
amplification of a 271-base pair fragment of the DRB3 gene from genomic
DNA, followed by membrane hybridization with a DRB3*0101 allele-specific digoxigenin-labeled oligonucleotide
probe.23
Statistics.
Fisher's exact test was used to analyze the associations between
HPA-1a alloimmunization and HLA-DRB3*0101 genotype, and titer of
anti-HPA-1a antibodies and cord blood platelet count, and to compare
the frequency of severe thrombocytopenia (<50 × 109/L) in babies of HPA-1a alloimmunized and
nonalloimmunized women. For the first two associations calculations
were made of odds ratios with confidence intervals, and positive and
negative predictive values.24 P < .05 was
considered significant.
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RESULTS |
Study group.
Of 25,379 consecutive blood specimens, 962 (3.8%) could not be typed
for HPA-1a because clotted blood samples were received. Of the
remaining 24,417 women, 618 (2.5%) were typed as HPA-1a negative, of
whom 457 (74%) consented to join the study. Seventy-two women were
subsequently excluded for various reasons: confirmatory HPA-1a testing
by PIFT showed them to be HPA-1a positive (20), incomplete or no blood
samples were available (40), or they suffered a miscarriage (8), moved
abroad (2), or requested removal from the study (2). Two further women
were included, who were not originally typed as HPA-1a negative, but
who had affected children and who were shown on retesting to be HPA-1a
negative. The final study population thus comprised 387 women, of whom
55% were multiparous.
Maternal serology.
HPA-1a antibodies were detected in 46 women (12%), including 2 (SL and
LR ) not originally typed as HPA-1a negative
(Table 1). Antibodies were
first detected at or before 20 weeks in 27 pregnancies, between 21 and
32 weeks in eight, between 33 weeks and term in three, and postnatally
in seven. No primigravida had detectable antibodies to HPA-1a before 17 weeks. Antibodies present before 20 weeks became undetectable until the
postnatal period in six pregnancies (transient antibodies); antibody
titers increased with time in five pregnancies, were stable in 12, and
decreased in 10. Neither antibody titer nor fluctuations therein was a
reliable predictor of an HPA-1a-positive fetus. None of the eight
women who suffered miscarriage had HPA-1a antibodies.
Risk factors for alloimmunization.
The presence of antibodies to HPA-1a was associated with HLA-DRB3*0101
(Table 2), with a positive predictive value
of 35% and a negative predictive value of 99.6%. The overall
frequency of this allele (31.9%) was as expected for a largely white
population.
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Table 2.
Association of Maternal Antibodies to HPA-1a With the
HLA DRB3*0101 Allele for the 385 Women in the Study Population
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Eight out of 33 women (25.8%) with antenatal antibodies to HPA-1a and
an HPA-1a-positive infant were primigravidas. In only 1 woman (USN
230) had neonatal alloimmune thrombocytopenia been diagnosed in her
previous child. No other cases reported affected infants, or
unexplained ICH/IUT. Other women may have been exposed to the HPA-1a
antigen during previous pregnancy (29), spontaneous or therapeutic
abortion (15), or blood transfusion (2), with some having more than one
risk factor. One woman (USN 487, para 3 + 1) underwent amniocentesis at
16 weeks, but antibodies to HPA-1a were present in the first sample
sent at 8 weeks.
Thrombocytopenia in infants of alloimmunized and apparently
nonalloimmunized women.
Of 26 HPA-1a-positive infants born to women with persistent antenatal
antibodies to HPA-1a, 9 were severely thrombocytopenic (34.6%),
including 1 (LR) not originally typed as HPA-1a negative (Table 3). Ten infants were
mildly thrombocytopenic (38.4%), and 7 had normal platelet counts.
Severe thrombocytopenia was not observed in association with transient
or postnatal antibodies, although cord samples were not received from 1 infant in each category. Thrombocytopenia (39-149 × 109/L) in cord samples was found in 12 of 237 infants of
HPA-1a-negative women in whom no HPA-1a antibodies were detected in
the initial MAIPA. Supplementary testing of sera from these 12 women
using both MAIPA with an alternative GPIIb/IIIa capture antibody (MoAb Y2.51) and the solid-phase GTI assay likewise failed to detect HPA-1a
antibodies. All 12 women were HLA-DRB3*0101 negative. In three cases,
another risk factor for thrombocytopenia was present (one case each
with antibodies to HPA-5b, extreme prematurity and respiratory distress
syndrome in a set of twins, and a GPIIb/IIIa antibody reactive with all
panel cells). No other risk factors were identified in the remaining
nine cases, but in all nine, the platelet count was normal on venous
sampling by 1 week.
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Table 3.
Cord Blood Platelet Counts for HPA-1a-Positive Infants
of Alloimmunized Women and All Infants of Nonalloimmunized Women and
Normal Controls
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In 25 HPA-1a-positive infants tested, the cord blood platelet count
was highly associated with the third trimester titer of antibodies to
HPA-1a in the MAIPA assay (Table 4). A
titer of 1 in 32 or greater had a positive-predictive value for severe thrombocytopenia of 75% and a negative-predictive value of 88%. Antibody titers taken earlier in the pregnancy were not a reliable indicator of severe thrombocytopenia. There was no clear relationship between antibody titer and parity. Although six of eight women with
high third trimester antibody titers were multiparous, this was also
true of 10 of 17 women with low titer antibodies. Similarly, severity
did not clearly correlate with parity, with 11 of 17 mildly/unaffected
and 5 of 10 severely affected cases being multiparous.
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Table 4.
Relation of Titer of Anti-HPA-1a at Different Stages of
Pregnancy to Cord Blood Platelet Count for Antenatally Alloimmunized
Women With HPA-1a-Positive Infants
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Clinical outcome of alloimmunized pregnancies.
Forty-four of the 47 alloimmunized pregnancies resulted in live births
at 36 weeks, with 19 spontaneous deliveries, 5 inductions of labor,
and 14 Caesarean sections (overall Caesarean rate, 36%) among the 38 live births of babies to women with antenatal antibodies.
Three pregnancies ended in loss of the baby, one at 15 weeks (USN 302)
and one as a neonatal death from immaturity after Caesarean section at
25 weeks for severe pre-eclampsia (USN 152, first pregnancy). The
third, in a woman not originally typed as HPA-1a negative (SL),
resulted in intrauterine death at 29 weeks due to hemorrhage from the
cord after blood sampling for investigation of unexplained fetal
hydrops. The baby's hemoglobin was 6.1 g/dL, with no detectable red-cell alloantibodies, and, unexpectedly, the platelet count was 6 × 109/L. The father was HPA-1a homozygous, and the
titer of maternal anti-HPA-1a was 1:128.
Only one of the remaining nine severely thrombocytopenic infants of
alloimmunized women showed evidence of major hemorrhage (infant of USN
347). This infant was delivered by emergency Caesarean section at 37 weeks because of an abnormal heart trace during labor. Cerebral
ultrasound scanning on postnatal day one showed a large left
occipitoparietal porencephalic cyst with encephalomalacia, consistent
with previous hemorrhage. The development of hydrocephalus in this
infant required a Rickman reservoir and, later, a ventriculoperitoneal shunt. He subsequently developed infantile spasms controlled on Vigabatrin (Hoechst Marion Rousel Limited, Uxbridge, UK),
with peripheral hypertonia, delayed motor and social development, and mild optic atrophy. The remaining infants had petechiae and/or bruising but no other hemorrhage. Cerebral scans of all other babies of
mothers with persistent antibodies to HPA-1a, including those with
severe thrombocytopenia, as well as of 49 babies of nonalloimmunized
DRB3*0101 positive women, were normal.
The stratified risk of HPA-1a alloimmunization and its complications
for different patient populations are shown in
Table 5.
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Table 5.
Stratified Risk (95% Confidence Interval) of HPA-1a
Alloimmunization and its Complications in At-Risk Populations
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Postnatal recovery.
The recovery of platelet counts in severely thrombocytopenic infants
born to alloimmunized mothers was variable. Two infants rapidly
recovered without platelet transfusion, with normal platelet counts by
postnatal days 6 and 8, whereas a further three babies received a
single infusion of HPA-1a-negative donor platelets and recovered
normal platelet counts after 2, 4, and 10 days. Multiple platelet
transfusions and intravenous immunoglobulin (Sandoglobulin 1 g/kg to 2 g/kg) were provided to the remaining three infants, whose platelet
counts required 9, 47 (USN 421), and 82 (USN 347) days to recover. The
delay in recovery of platelet counts may relate to the presence of
intracerebral hemorrhage in USN 347, but is unexplained in USN 421.
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DISCUSSION |
Our study has provided baseline data on the natural history of HPA-1a
alloimmunization in a cohort of 387 HPA-1a-negative pregnant women,
equivalent to approximately 15,000 nonselected pregnancies. In contrast
to previous studies with a smaller sample size, this study is of
sufficient size to provide statistically reliable data on the incidence
of alloimmunization and the effect of anti-HPA-1a antibody on the
neonatal platelet count, unbiased by preselection of high-risk
pregnancies. It must be considered whether the 75% of HPA-1a-negative
women who enrolled for the study were in any way unrepresentative of
the whole population. The predominantly white nature of the East
Anglian population masks any small effect of ethnic bias, with
frequencies of HPA-1a negativity and HLA-DRB3*0101 positivity in the
study population as expected for whites. The proportion of multiparous
women in the study population (55%) is as expected for this region
(data from Rosie Maternity Hospital, Cambridge, UK). Finally, only 1 of
the study population reported a previously affected child.
Fetomaternal alloimmunization to HPA-1a was apparent in approximately 1 in 350 pregnancies, corresponding to 11.4% of HPA-1a-negative women.
This rate is higher than that observed in most previous studies
(0%,16 1.4%,15 and 6%13) but
similar to that in another study of 45 HPA-1a-negative women (10%).14
Although the association between HLA-DRB3*0101 positivity and HPA-1a
alloimmunization has previously been observed in clinically apparent
NAITP,25 the negative predictive value of greater than 99%
has helped to clarify its potential usefulness in an unselected white
population. We observed only one case of alloimmunization in a woman
negative for this allele, as previously observed.5 Although
the presence of the HLA-DRB3*0101 allele increased the risk of
alloimmunization in HPA-1a-negative women by a factor of 140, its
positive-predictive value as a single marker was only 35%.
Maternal parity was not highly predictive of alloimmunization, with
25% of antibody-positive cases in primiparous women, in whom
antibodies were detectable as early as 17 weeks, consistent with
development of antigens of fetal platelets26 and expression of the  3 integrin on syncytiotrophoblast.27 Previous
pregnancy loss before 20 weeks of gestation also appeared to be an
potentially immunizing event, with detection of antibodies early in the
next pregnancy in two cases. Serial antibody titers showed considerable variation during the evolution of the antibody response during the
course of the pregnancy, as previously observed.28 In some women, antibodies that were clearly present early in the pregnancy later became undetectable (transient antibodies), only to emerge again
clearly in the postnatal period. The pattern of antibody response was
not a reliable predictor of fetal HPA-1a type.
This is the first prospective study of sufficient size to be able to
estimate the risk to the fetus due to the presence of maternal HPA-1a
alloantibodies, in which there has not been a previously diagnosed
sibling. Although the risk of severe disease in siblings of affected
cases is high,29 the pathogenicity of HPA-1a alloantibodies
in an unselected population is highly variable. First, 15% of babies
born to HPA-1a-negative women are themselves HPA-1a negative, and thus
unaffected. Second, even with persistent antenatal antibodies and an
HPA-1a-positive fetus, a normal cord platelet count was observed in 7 of 26 cases (35%), and a `safe' platelet count (between 50-150 × 109/L30) in a further 10 (38%). Neither cord platelet count nor final antibody titer could be
reliably predicted by either parity or timing of immunization, but we
have observed for the first time a significant association between
severe thrombocytopenia and a third trimester antibody titer of 1:32 or
greater using the MAIPA assay, with a positive-predictive value of
75%.
Although two previous studies have not found antibody titer to be a
predictor of disease severity,31,32 they lacked the power
of this larger series, whereas the second of these measured titers only
in the postnatal period, and used an ELISA.32 Our findings
are, however, consistent with previous data using the MAIPA technique,
in which absorbance values (rather than titer) correlated with infant
platelet count.28 Conversely, we observed two infants with
mild thrombocytopenia born to mothers with high-titer antibodies; this
phenomenon occurs in hemolytic disease of the newborn, and is possibly
attributable to maternal IgG antibodies to paternal HLA class-II
antigens on fetal monocytes.33 We found neither transient
nor postnatal antibodies to be associated with severe thrombocytopenia,
although one case classified as mild (USN 412, platelets 52 × 109/L) had postnatal antibodies only.
Thrombocytopenia in infants of HPA-1a-negative women without
detectable HPA-1a antibodies is well recognized, and is generally assumed to be due to failure of antibody detection. We found mild thrombocytopenia in 13 infants (including 1 set of twins) of 249 HPA-1a-negative mothers (4.8%). Four of these 12 infants had other potential causes, including alloantibodies to HPA-5b34 and
antibodies to GPIIb/IIIa. Extra efforts were made to detect HPA-1a
antibodies in these women, using two additional antibody detection
assays, but both assays gave negative results in all 12 cases. A point
of note is that all 12 women were HLA-DRB3*0101 negative. This HLA
distribution must cast some doubt on whether these cases were due to
HPA-1a antibodies, because the importance of binding between
3-derived peptides and HLA-DRB3*0101 molecules in the immune
response to HPA-1a has now been shown.35 It is of interest
that we saw no antibody-negative thrombocytopenic cases in infants of
HLA-DRB3*0101-positive women, as might have been expected. However,
our observations do not exclude the possibility that in the eight
unexplained cases, alloimmunization to other as yet unidentified
platelet alloantigens play a role. A further possibility is spurious
thrombocytopenia from cord sampling, as the 200 normal controls came,
for unavoidable logistical reasons, from venous samples.
There was one fatality most likely due to HPA-1a alloimmunization in
our study (patient SL), after diagnostic fetal blood sampling for
hydrops fetalis. In this case, the severe fetal thrombocytopenia was
unexpected, so platelets were not made available during the procedure.
Another infant (of 38-weeks gestation) sustained an antenatal
intracerebral hemorrhage (ICH). No additional cases of intracerebral
hemorrhage were observed postnatally, but the preventive role of
altered obstetric management (lower threshold for Caesarean section)
combined with early postnatal platelet transfusion therapy cannot be
excluded. There were no obvious predictors of intracerebral hemorrhage
in the affected child, the third trimester antibody titer being 1 in
64, as in four other infants without ICH.
Antenatal screening for platelet-specific alloantibodies is not a
standard of care for women with no previous history of NAITP. Several
observations from this study will help in evaluating different strategies for cost-effectiveness and patient acceptability. For example, HLA DRB3*0101 genotyping could be used to exclude low-risk women from further antibody screening, thus reducing both unnecessary testing and parental anxiety. Antibodies detected before 20 weeks require confirmation with a later specimen, because early transient antibodies are of no clinical significance. Screening will be of value
only if a treatment plan can be implemented that prevents ICH and fetal
loss, with minimum harm to fetuses who may in any case be unaffected by
maternal antibodies. Had infants of all antenatally immunized mothers
in this study been subjected to fetal blood sampling, 23 of 33 infants
would have been exposed unnecessarily to the risks of the procedure,
some perhaps more than once. Even if women with transient antibodies
and those with an HPA-1a-negative fetus had been excluded, 17 of 27 samplings would have been unnecessary. Apart from the risks to the
fetus, the resource implications of such a strategy are considerable. Antenatal cerebral scanning to look for early ICH has not been evaluated as a means of preventing major bleeds. Postnatal screening has been advocated, measuring cord blood platelet counts in all neonates,36 but this has been shown not to prevent all
cases of ICH.14 Although the laboratory costs of postnatal
screening are less than antenatal testing,14 this strategy
may cease to be cost-effective if the lifetime care costs of missed ICH
cases are taken into account. Other problems with neonatal screening include potential nonavailability out of hours, and the observation that as many as 7% of cord samples are unsuitable for
counting.14 The additional clinical benefit of detecting
thrombocytopenia due to maternal autoantibodies36 remains
to be quantified.
As yet, the best strategy for antenatal management of alloimmunized
cases is unclear. Of the options for antenatal therapy, intravenous
immunoglobulin is of unpredictable efficacy, whereas intrauterine
platelet transfusion is associated with a small but finite risk of
fetal loss, mainly due to cord hemorrhage (0.6% in one
study37). Noninvasive management exposes fetuses to the risk of intruterine ICH or death. The knowledge that HPA-1a antibodies are present may also increase the incidence of Caesarean
section (36% in this study from a baseline value of 15%). This has
unproven benefit to the fetus and carries its own risks, albeit low, to the mother. Thus preselection of high-risk cases for fetal blood sampling remains in our view the ideal option. Our data on the predictive value of antibody titer offer the possibility of avoiding fetal sampling in the two-thirds of alloimmunized pregnancies with a
safe platelet count. The exact role of the inhibitory effects of HPA-1a
antibodies on platelet progenitors in culture38 and on the
binding of ligands to GPIIb/IIIa in the clinical manifestations of
NAITP remain to be elucidated.
The technical aspects of NAITP screening require further refinement, as
shown by the initial mistyping of two cases. This phenomenon did not
impact on the overall frequency of HPA-1a negativity, which was
identical to that of a smaller UK study using a different typing
assay.15 It is acceptable for screening assays to have a
false-positivity rate, provided that a confirmatory test is available.
This could be applied to HPA-1a typing, with the cut off set
sufficiently high to prevent any HPA-1a-negative cases from being
missed, and rapid confirmation of apparent negatives by an alternative
assay. However, this is less than ideal, as is the need to prepare
platelet rich plasma before typing, which reduces scope for automation
and positive sample identification. The availability of a whole-blood
typing assay for HPA-1a39 and a human monoclonal variable
domain antibody fragment specific for HPA-1a40 may
facilitate large-scale screening programs. As was the case with Rhesus
D typing in antenatal screening programs, it is envisaged that the
reliability of typing will improve once monoclonal reagents are
introduced.
The optimal screening strategy for FAIT screening remains unclear.
Future studies of screening will have to be sufficiently large to
ensure that any true impact on long-term outcome can be statistically
detected, and appropriately costed.
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FOOTNOTES |
Submitted September 8, 1997;
accepted May 18, 1998.
Supported by a grant from the East Anglian Regional Health Authority,
Headington, Oxford, UK.
Address correspondence to Lorna M. Williamson, MD, Division of
Transfusion Medicine, University of Cambridge, Long Road, Cambridge, CB2 2PT, UK; e-mail: lorna.williamson{at}nbs.nhs.uk.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" is accordance with 18 U.S.C. section 1734 solely to indicate this fact.
| |
ACKNOWLEDGMENT |
We thank the many obstetric, midwifery, paediatric, and blood bank
staff who contributed to this study; C. Holmes, C. Milnes, S. Chisholm,
A. Pollock, J. Walton, and R. Fagence for help with clinical liaison
and provision of HPA-1a-negative platelets; R. Lambert, M. Regtuijt,
C. Hurd, and the tissue-typing staff of West Midlands Blood Centre for
laboratory support; and J-P. Allain for constructive criticism.
| |
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Abstract
Introduction
Abstract
