Red cell isoimmunisation

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Chapter 31 Red cell isoimmunisation

Perinatal mortality rate

Perinatal mortality due to Rhesus D (RhD) isoimmunisation before the use of prophylaxis was 15 in 1000, or 4% of perinatal deaths. Today, the perinatal mortality rate from RhD isoimmunisation is 0.54 in 10,000, or 0.3%. The reduced perinatal mortality is due to anti-D immunoglobulin prophylaxis and the fall in numbers of large families.

A third of women who become isoimmunised have no demonstrable antibodies at the first antenatal visit, so investigations for antibodies must be repeated during pregnancy in RhD-negative women.

There is a correlation between the time of first antibody detection and the severity of haemolytic disease of the newborn. The earlier the antibodies are present during the pregnancy, the higher the risk of fetal/neonatal problems. If an antibody is present in pregnancy, serial monitoring of antibody levels is required.

Prevention of RhD isoimmunisation

The aim is to provide passive immunisation at times of risk of fetomaternal transfusion. If no prophylaxis is given, 1% of RhD-negative women will develop anti-D antibodies by the end of the first pregnancy and a further 3%–5% have detectable antibodies 6 months after delivery. About 90% of sensitisations can be prevented. 100 IU anti-D immunoglobulin is able to neutralise 2 mL fetal blood. This is given within 72 hours of fetomaternal transfusion for maximum effect. In an RhD-negative woman with no anti-D antibodies present, anti-D prophylaxis is indicated in the following cases:

• Sensitising events in the first trimester (including miscarriage, ectopic pregnancy, termination of pregnancy, chorionic villus sampling) require 250 IU intramuscular RhD immunoglobulin for single pregnancy and 625 IU for multiple pregnancy.

• Events in the second and third trimester (e.g. amniocentesis, cordocentesis, abdominal trauma sufficient to cause fetomaternal haemorrhage, external cephalic version or antepartum haemorrhage) require 625 IU injection of anti-D.

• There is routine administration of 625 IU at 28 and 34 weeks gestation in addition to doses given for sensitising events.

• Postpartum administration of 600 IU is necessary within 72 hours.

• With this prophylaxis regimen, antibody formation rate is reduced to 0.2%.

Contraindications for anti-D prophylaxis

• RhD-positive or Du-positive individual

• RhD-negative or Du-negative individual previously sensitised to RhD

Prevention of postpartum isoimmunisation

• With anti-D prophylaxis, only 0.2% are sensitised.

• In 0.5% of deliveries, a fetomaternal haemorrhage of over 30 mL occurs.

• There is an increased risk with manual removal of the placenta and caesarean sections.

Screening

• All pregnant women have a red cell antibody screen in the first trimester and, when negative, again at around 28–30 weeks gestation.

• When RhD antibody is present, screen the partner’s blood to detect the likelihood of the fetus carrying D antigen. If the partner is RhD-negative, the fetus will be unaffected.

Antenatal management of Rhesus isoimmunisation

Aims

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