Anti-D: preventing rhesus isoimmunisation
Midwives have a key role in preventing rhesus isoimmunisation. This occurs when a rhesus-negative mother develops antibodies, which may subsequently cause destruction of the red blood cells of a rhesus-positive fetus. Timely use of anti-D has been very successful in reducing perinatal death and morbidity associated with isoimmunisation. A maternal blood test can identify the fetal blood type during pregnancy and this will allow a more targeted approach for anti-D administration.
Rhesus factor (Rh D) is one of the substances used to classify human blood with regard to its compatibility for transfusion. Individuals are classified as either rhesus positive (Rh D antigen present) or rhesus negative (Rh D antigen not present) (Figure 81.1). Rhesus isoimmunisation occurs when a mother’s blood type is rhesus negative and her fetus’s blood type is Rh D positive (inherited from the father).
The placenta normally acts as a barrier to fetal blood cells entering the maternal circulation. However, during late pregnancy, following a potentially sensitising event (PSE) (Box 81.1) but particularly at birth with the separation of the placenta, small amounts of fetal blood can enter the maternal circulation. This is known as a fetal maternal haemorrhage (FMH). The Rh D-negative mother’s immune system mounts an immune response to the fetal Rh D-positive cells in her system and produces Rh D antibodies. This process is called isoimmunisation, also known as sensitisation, and once sensitisation occurs it cannot be reversed (Figure 81.2).
The antibodies do not normally affect the current pregnancy but subsequent pregnancies, where the mother is carrying a rhesus-positive baby, then the antibodies will cross the placenta and attack the fetal red blood cells. This can cause haemolytic disease of fetus or newborn (HDFN). Features of HDFN include anaemia and jaundice, which may result in significant perinatal morbidity or mortality.
The exact mechanism of how anti-D works is unclear but it appears the passive anti-D binds to any fetal Rh D-positive cells and removes them from the maternal circulation before the mother’s immune system can trigger antibody production. There are two strategies to prevent rhesus isoimmunisation with administration of anti-D:
- Anti-D administration of a rhesus-negative mother following a sensitising event, predominantly the birth of a rhesus-positive infant but also following a known PSE in pregnancy (Box 81.1)
- Routine antenatal anti-D prophylaxis (RAADP) most commonly given as a one-dose injection of anti-D (1500 i.u. IM) at 28–30 weeks of pregnancy. The anti-D is given as a preventative measure to cover the possibility of an unknown FMH during the last trimester of pregnancy.
Anti-D is a prescription drug, included under the midwives exemptions allowing midwives to administer it without a prescription. The correct timing of administration, the correct dose and giving it to the correct woman is essential. The correct dose depends on the stage of the pregnancy and the size of the FMH and the correct time requires that anti-D be administered as soon as possible, but within 72 hours, of birth or any other PSE. Intramuscular anti-D should be given into the deltoid muscle (Figure 81.3) as injections into the gluteal region often only reach the subcutaneous tissues and absorption may be delayed.
Anti-D is a blood product and as such there is a small risk of the transmission of blood-borne infections. There is also a risk of an allergic reaction although these are rare. The midwife should therefore provide the woman with both written and verbal information prior to obtaining consent for administration. Some women may choose to decline anti-D for religious reasons, if they anticipate only having children with a partner known to be rhesus negative, or if she is not intending any further children. The woman’s reason for declining should be discussed and documented.
A clear audit trail of anti-D administration is required with details of the prescriber/midwife, the batch number, the dose, route, date and time of administration recorded. Local protocols for administration should be followed as product and dose may vary, although national guidelines from Serious Hazards of Transfusion (SHOT) and the British Committee for Standards in Haematology (BCSH) are recommended.
Box 81.2 outlines maternal and cord blood tests.
The Kleihauer test on maternal blood is done after birth and after any PSE to determine the extent of fetal–maternal haemorrhage. A standard dose of anti-D (500 i.u. IM) is usually enough to cover a bleed of up to 4 mL and anti-D should be given as soon as possible after birth or any PSE, and within 72 hours, without waiting for the test result.
Box 81.3 list situations that have been associated with a larger FMH. In very large FMH (greater than 100 mL) intravenous anti-D should be considered.
SHOT (Serious Hazards of Transfusion) reports a rise in errors relating to the use of anti-D, mostly due to omission or late administration of anti-D. Reports suggest:
- Lack of knowledge of when and how anti-D should be administered, although factors such as understaffing and the rapid turnover of women, probably play a part
- Misuse and misinterpretation of Kleihauer test, (performed solely to determine if more than the standard dose is required)
- Confusion between the administration of RAADP and in response to a sensitising event.
Anti-D must still be administered in response to a PSE, even if the woman has received or is due to receive her RAADP and vice versa.