38.  Screening for D (Rh) Incompatibility



RECOMMENDATION



D (formerly Rh) blood typing and antibody screening is recommended for all pregnant women at

their first prenatal visit. Repeat antibody screening at 24-28 weeks' gestation is recommended

for unsensitized D-negative women (see Clinical Intervention).



Burden of Suffering



D incompatibility exists when a D-negative woman is pregnant with a D-positive fetus, which

occurs in up to 9-10% of pregnancies, depending on race.1,2 If no preventive measures are

taken, 0.7-1.8% of these women will become isoimmunized antenatally, developing D antibody

through exposure to fetal blood; 8-17% will become isoimmunized at delivery, 3-6% after

spontaneous or elective abortion, and 2-5% after amniocentesis.1-3 In subsequent D-positive

pregnancies of isoimmunized women, maternal D antibody will cross the placenta into the fetal

circulation and hemolyze red cells.1 Without treatment, 25-30% of these offspring will have

some degree of hemolytic anemia and hyperbilirubinemia, and another 20-25% will be hydropic

and often will die either in utero or in the neonatal period.4



Since the introduction of routine postpartum prophylaxis in the 1960s, the crude incidence of D

isoimmunization in the U.S. and Canada has fallen from 9.1-10.3 cases to 1.3 cases/1,000 total

births.5-9 Hemolytic disease of the fetus or newborn due to D isoimmunization (also called

erythroblastosis fetalis) now accounts for only 4-5 deaths/100,000 total births,6,10 although

this may be an underestimate as early intrauterine deaths are not always reported.10 Even before

the introduction of prophylaxis, however, a decline in fetal and neonatal mortality from D

hemolytic disease was occurring due to declines in both incidence and case fatality rates. It has

been estimated that 30-40% of the recent decline in disease incidence is attributable to smaller

family size, since the incidence of D hemolytic disease increases with increasing birth order.11

Since the 1940s, the case fatality rate has fallen from about 50% to 2-6%.8,9 This decline can

be attributed in part to the trend toward smaller families, since the first affected infant in a

family generally has less severe disease.1,5,9 The decline has also been associated with the

introduction of interventions such as amniotic fluid spectrophotometry, exchange transfusion,

amniocentesis, intrauterine fetal transfusion, and improved care of both the mother and the

premature erythroblastotic infant.1,9



Accuracy of Screening Tests



Hemagglutination is the established reference standard for the determination of D blood type.12

The indirect antiglobulin (Coombs) test (IAGT) is the reference standard for detecting anti-D

antibody in women who are sensitized to D-positive blood.12 The IAGT will also detect other

maternal antibodies that may cause hemolytic disease.13



Effectiveness of Early Detection



The early detection of D-negative blood type in the pregnant woman is of substantial benefit if the

patient is not yet isoimmunized and the father is not known to be D-negative. Administration of D

immunoglobulin (or Rho(D) immune globulin (human)) to an unsensitized D-negative woman

after delivery of a D-positive fetus will prevent maternal isoimmunization and consequent

hemolytic disease in subsequent D-positive offspring. The efficacy of D immunoglobulin

prophylaxis was convincingly demonstrated in a series of controlled clinical trials in the early

1960s.14-16 Despite a variety of minor flaws in study design, these trials showed that

isoimmunization did not occur in any of the women who received a full dose of D immunoglobulin

postpartum and who were unsensitized when it was administered. These findings led to the

introduction of routine postpartum prophylaxis following licensure of D immunoglobulin in 1968.

Time series studies have since shown a dramatic decline in the incidence of D isoimmunization,

from 13-14% in the mid-1960s to 1-2% in the mid-1970s,7,17 although as described above,

at least some of this decline is probably attributable to smaller family size.



The most frequent cause of apparent failure of postpartum prophylaxis is antenatal

isoimmunization, which happens in 0.7-1.8% of pregnant women at risk.1,9,18 Although sample

selection and other design features were not optimal, nonrandomized controlled trials have shown

that the administration of D immunoglobulin at 28 weeks' gestation, when combined with

postpartum administration, reduces the incidence of isoimmunization to 0.2% of women at

risk.19-21



Since D isoimmunization during pregnancy is caused by transplacental hemorrhage, the risk of

isoimmunization increases whenever such hemorrhage is likely to occur, including after abortion,

amniocentesis, chorionic villus sampling (CVS), cordocentesis, ectopic pregnancy, fetal

manipulation (e.g., external version procedures) or surgery, antepartum hemorrhage,

antepartum fetal death, and stillbirth.1,22-24 Studies documenting the effectiveness of D

immunoglobulin prophylaxis are available for only a few of these indications, however. In a

nonrandomized trial of D immunoglobulin after amniocentesis, control D-negative women

delivering D-positive infants were more likely to become isoimmunized than were those receiving

D immunoglobulin (5.2% vs. 0%), although because of small numbers this difference was not

statistically significant.25 Case series describing D immunoglobulin administration after

amniocentesis have demonstrated isoimmunization rates as low as 0-0.5%.26-28 In a case series

of D immunoglobulin after induced abortion, isoimmunization occurred in 0.4%,29 compared to

2.6% among a series of patients, described by the same authors, who did not receive D

immunoglobulin.30 The preliminary results from a randomized controlled trial of D

immunoglobulin after CVS showed that among D-negative women delivering D-positive infants,

similar rates of isoimmunization were seen in both intervention (2.3%) and control (1.1%)

groups; insufficient details are provided to ensure baseline comparability between the two groups,

however.31 D-negative women who received D immunoglobulin experienced twice as many

unintended fetal losses as did controls (6.9% vs. 3.8%), but this difference was not statistically

significant. Results of the completed trial confirm the preliminary findings (S. Smidt-Jensen,

Rigshospitalet, Copenhagen, Denmark; personal communication, January  1995), but have not yet

been published. No studies evaluating the use of D immunoglobulin after other obstetric

procedures or after obstetric complications werefound.



The standard postpartum dose of D immunoglobulin (300 mg) contains sufficient D antibodies to

prevent sensitization to at least 15 mL of D-positive fetal red blood cells (RBCs), or

approximately 30 mL of fetal blood;32 a "minidose" (50 mg) prevents sensitization to 2.5 mL of

D-positive fetal RBCs. For women with transplacental hemorrhages >30 mL of fetal blood, the risk

of D isoimmunization developing after the full postpartum D immunoglobulin dose is 30-

35%.3,33 The incidence of fetal-maternal hemorrhage >30 mL is 0.1-0.7% for all D-negative

pregnancies,1,33,34 but it is 1.7-2.5% after complicated vaginal and cesarean deliveries,34,35

and 4.5% after stillbirth.1 There are several available methods for detecting excess fetomaternal

hemorrhage. Acid elution (Kleihauer-Betke) is both sensitive and specific when done

correctly,1,36 but it is subject to substantial laboratory and technologist error.36 Flow

cytometry is also highly sensitive and specific, but it is technically difficult to perform.36 The

erythrocyte rosette test is simple to perform and highly sensitive (99-100%) for the presence

of 15 mL of D-positive fetal RBCs,1,36 but its specificity is low36,37 so positive results must

be confirmed by more specific tests such as acid elution and flow cytometry.1,36



In clinical practice, combined antenatal and postnatal prophylaxis will prevent isoimmunization

in 96% of women at risk.21 The remaining cases are due to failure to give D immunoglobulin

when indicated, isoimmunization that occurred before the widespread availability of D

immunoglobulin, administration of an insufficient dose, or treatment failure (i.e.,

isoimmunization occurring before 28 weeks or transplacental hemorrhage too large or too late in

pregnancy to be prevented by the standard antepartum dose).3,38,39 Human error causes 22-

50% of these cases.6,21,39 While clinicians almost always administer D immunoglobulin

postpartum or after induced abortion, administration rates have been documented to be lower for

other obstetric procedures and complications: 81-88% after spontaneous abortion, 36-60%

after ectopic pregnancy, 31% after antepartum hemorrhage, and 14% after

amniocentesis.2,40,41



D immunoglobulin has few adverse effects.1,42 Some fetuses will become weakly direct

antiglobulin-positive following antenatal administration, but resulting anemia and

hyperbilirubinemia in the newborn are very rare.19 All plasma for D immunoglobulin production

is screened for infectious diseases as required by the Food and Drug Administration; no cases of

human immunodeficiency virus (HIV) infection from D immunoglobulin have been reported.43

The evidence is therefore compelling that early detection and prophylaxis of the unsensitized

D-negative woman is both safe and effective in preventing isoimmunization and thus in preventing

D hemolytic disease.



Early detection is also beneficial for D-negative women who are already isoimmunized and are

carrying D-positive offspring, because early intervention may improve clinical outcome.

Decisions to intervene depend on the validity of screening tests in predicting the degree of fetal

anemia. Obstetric history, maternal antibody titers, and ultrasound are currently used to

determine the need for more invasive tests during isoimmunized pregnancies, but in the absence of

hydrops none of these reliably distinguishes mild from severe hemolytic disease.1,4,22,44

Immunologic tests on maternal serum show promise in predicting disease severity.1,45,46 In the

third trimester, serial amniotic fluid spectrophotometry has been found to correctly predict

disease severity (i.e., cord hemoglobin and need for neonatal therapy) in 94-99% of cases.47,48

In the second trimester, however, this test has insufficient sensitivity or specificity for

predicting the need for intervention.4,49,50 Determination of fetal hemoglobin and D blood type

by ultrasound-guided cordocentesis, which can be performed in the second trimester, quantifies

the degree of anemia, can be followed by transfusion if indicated, and allows referral of those with

D-negative babies to routine care.1,4 Case series, however, have demonstrated complication rates

of 2-7% and procedure-related fetal mortality rates of 0.5-1%.23,51,52 DNA amplification in

amniotic cells and chorionic villus samples appears to be effective in determining fetal D blood

type early in pregnancy, without the risk associated with invading the fetomaternal circulation.53



In the presence of severe fetal anemia, early intervention appears to offer substantial

improvement in clinical outcome. Current perinatal survival after ultrasound-guided

intravascular transfusion at experienced centers is 62-86% for hydropic fetuses and >90% for

those without hydrops.4,54,55 Once pulmonary maturity is established, the fetus can be delivered

early and exchange transfusion performed with only 1% mortality risk.56



Recommendations of Other Groups



The American College of Obstetricians and Gynecologists (ACOG)22 and the U.S. Public Health

Service Expert Panel on the Content of Prenatal Care57 recommend D blood typing and antibody

screening at the first prenatal visit and repeat D antibody screening at 24-28 weeks of pregnancy

for D-negative women. Both groups recommend offering D immunoglobulin to all unsensitized

D-negative women at 28 weeks of gestation, and to those at increased risk of sensitization because

of delivery of a D-positive infant, antepartum hemorrhage, spontaneous or induced abortion,

amniocentesis, external version procedures, or ectopic pregnancy, within 72 hours of the

event.22,57 ACOG also recommends D immunoglobulin administration to unsensitized D-negative

women who have CVS, cordocentesis, antepartum fetal death, fetal surgery, or transfusion of

D-positive blood products.22 ACOG recommends measuring fetal blood cell levels in the mother

when antepartum placental hemorrhage occurs.22 The Canadian Task Force on the Periodic Health

Examination recommends D blood typing and antibody screening at the first prenatal visit, before

elective procedures such as amniocentesis and therapeutic abortion in which there is the

possibility of fetal bleed, between 24 and 28 weeks if the mother is D-negative, and within 72

hours of delivery. They recommend administration of D immunoglobulin to unsensitized women at

28 weeks and postpartum, and after amniocentesis or induced abortion.58



Discussion



Although the burden of suffering from this disease is now low, the incidence was at least

10/1,000 live births before the introduction of preventive measures in the 1960s.9 There is

excellent evidence for the efficacy and effectiveness of blood typing, anti-D antibody screening,

and postpartum D immunoglobulin prophylaxis. Although antepartum prophylaxis offers some

additional benefit, some critics argue that the total impact of antepartum prophylaxis on the

incidence of D disease is relatively small, making it approximately 16 times less cost-effective

than a program consisting only of postpartum treatment.2,59,60 Other studies support the

cost-effectiveness of antepartum prophylaxis.21,61 The cost-effectiveness of D immunoglobulin

after obstetric procedures and complications is unknown.



CLINICAL INTERVENTION



D blood typing and antibody testing is recommended for all pregnant women at their first prenatal

visit, including visits for elective abortion ("A" recommendation). For purposes of blood typing

and prophylaxis, Du- and D-negative blood types should be considered equivalent.22 Unless the

father is known to be D-negative, a repeat D antibody test is recommended for all unsensitized

D-negative women at 24-28 weeks' gestation, followed by the administration of a full (300 mg)

dose of D immunoglobulin if they are antibody-negative ("B" recommendation). If a D- (or Du-)

positive infant is delivered, the dose should be repeated postpartum, preferably within 72 hours

after delivery ("A" recommendation). Unless the father is known to be D-negative, a full dose of

D immunoglobulin is recommended for all unsensitized D-negative women after elective abortion

(50 mg before 13 weeks) and amniocentesis ("B" recommendation). There is currently

insufficient evidence to recommend for or against the routine administration of D immunoglobulin

after other obstetric procedures or complications such as chorionic villus sampling, ectopic

pregnancy termination, cordocentesis, fetal surgery or manipulation (including external

version), antepartum placental hemorrhage, antepartum fetal death, and stillbirth ("C"

recommendation).



The draft update of this chapter was prepared for the U.S. Preventive Services Task Force by

Carolyn DiGuiseppi, MD, MPH.



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