Antenatal Blood Group Serology

In the antenatal blood grouping and antibody screening programme, the BCPS complies with:

• Guidelines for Blood Grouping and Antibody Testing in Pregnancy (2016) https://b-sh.org.uk/guidelines/guidelines/blood-grouping-and-antibody-testing-in-pregnancy/
• Use of anti-D immunoglobulin for the prevention of Haemolytic Disease of the Fetus and Newborn https://b-s-h.org.uk/guidelines/guidelines/use-of-anti-d-immunoglobin-for-theprevention-of-haemolytic-disease-of-the-fetus-and-newborn/

The booking sample, generally taken from all pregnant women at 8-11+6 weeks gestation, is tested to establish the ABO and RhD (now called simply ‘D’) blood groups and for the presence of red cell antibodies. If no antibodies are detected a repeat sample for the same tests is requested at the start of the third trimester, ideally at 28 weeks gestation. Similarly, since many antibodies do not harm the baby, in cases where clinically non-significant antibodies are detected only one more test, at 28 weeks, is normally recommended. A very small number of women are found to have antibodies which are more likely to affect the baby, e.g., anti-D, anti-c and anti-K. More frequent samples will be requested if these red cell antibodies are present, or if there is any other cause for concern regarding the baby’s well-being.

The objectives for routine blood group and antibody screening in pregnancy are to:

• Identify D negative and D variant women who need anti-D immunoglobulin prophylaxis.
• Detect and identify red cell antibodies.
• Identify pregnancies at risk of fetal and neonatal haemolytic disease resulting from maternal antibodies crossing the placenta and entering the fetal circulation.

When red cell antibodies are present during pregnancy, the purposes of follow-up serological tests are to:

• Identify the fetuses that need monitoring and possibly treatment in the antenatal period.
• Predict which infants might require treatment and should be monitored more carefully after birth.
• Detect new antibodies, since those patients who develop one antibody are more likely to develop additional antibodies.
• Provide information to underpin medical advice on the management of the pregnancy.

If the antibody is confirmed and is of clinical significance to the fetus, the antibody will be quantified or titrated, and follow-up tests performed as recommended by BSH guidelines. Antibody cards with explanatory notes are issued for all women who have clinically significant antibodies.

The follow-up investigations are:

• Monitoring maternal red cell alloantibody levels.
• Identification of possible additional antibodies.
• Red cell phenotyping of the biological father when necessary.
• Fetal genotyping if required.

Haemolytic Disease of the Fetus and Newborn

Red cell antibodies are produced as a result of sensitisation due to a previous pregnancy, previous blood transfusion or, rarely, the current pregnancy. The current pregnancy will be affected only if the fetus has inherited the blood group antigen against which the mother has formed a clinically significant antibody against.

Haemolytic disease of the fetus and newborn (HDFN) occurs when antibodies from the mother’s circulation cross the placenta and bring about the destruction of the red blood cells of the fetus. Many red cell antibodies have the potential to cause HDFN. It is essential, therefore, for ALL pregnant women found to have clinically significant red cell antibodies to be referred to a hospital obstetric unit for management of their pregnancy.

Anti-D, anti-c and anti-K:

Although the majority of maternal IgG antibodies have been known to cause HDFN, the greatest risk is found with anti-D, anti-c and anti-K. In these cases, investigations and reports will include.

• Testing the mother’s blood at regular intervals according to the BSH guidelines.
• Red cell phenotyping the biological father.
• Advice about:
  – further samples required for antibody monitoring.
  – referral for fetal genotyping.
  – referral to a specialist fetal medicine unit.

HDFN is now so rare that at-risk pregnancies should be managed in specialist units to allow expertise to accumulate.

Haemolytic Disease of the Fetus and Newborn due to anti-D

Anti-D is formed in D negative women usually as a result of D positive fetal red cells crossing the placenta and entering the maternal circulation. The most common time this may happen is during delivery, but it is not uncommon for there to be silent leaks (with no pain or bleeding) of red cells from the fetus into the mother, especially in the third trimester. Anti-D prophylaxis, which aims to prevent the stimulation of maternal anti-D, has greatly reduced the frequency of HDFN due to anti-D.

However, anti-D still remains the most significant cause of HDFN. If anti-D and/or anti-c is detected in the maternal sample, quantification of the antibody is performed to predict the severity of HDFN. This sample will be referred to the NHS Blood and Transplant (NHSBT) reference laboratory for quantification.

Haemolytic Disease of the Fetus and Newborn due to antibodies other than anti-D

Many red cell antibodies other than anti-D have the potential to cause HDFN, though less commonly. The most important of these are anti-c and anti-K (Kell). Samples from women with these antibodies, and anti-D, are normally requested at 4 weekly intervals up until 28 weeks gestation and then at 2 weekly intervals to term. However, while anti-K is a significant cause of HDFN, in >90% of cases the partner is K negative.

A sample from the biological father of the pregnancy should be K typed and if negative, and if confirms paternity, samples from the mother will be requested for retesting at 28 weeks gestation. If the father is K positive or if there is any doubt of paternity, samples will be requested at the intervals given above. The reports issued on investigations on women with red cell antibodies include advice on their significance with respect to HDFN.

ABO Haemolytic Disease of the Newborn

HDFN sometimes occurs due to incompatibility between the ABO blood group of the mother and fetus. Laboratory tests during pregnancy cannot predict this condition but fortunately ABO HDFN s usually mild and the jaundice is easily treated with UV light (phototherapy). ABO HDFN usually presents with jaundice at around 48 hrs after birth.

Clinically non-significant antibodies

Many commonly occurring antibodies detected during pregnancy are of little clinical significance and do not cause HDFN. These are antibodies such as anti-A1, anti-HI, anti-P1 and the Lewis antibodies.

Measurement of maternal antibody levels during pregnancy (quantification and titration)

Measurement of antibody concentration depends on the antibody specificity:

• Anti-D and anti-c are quantified in comparison to a national standard.
• Other antibodies, including anti-K, are titrated by testing a series of doubling dilutions of the antibody. The result is expressed as a titre, i.e. the greatest dilution of plasma which gives a serological reaction. A titre of 32 or above is generally considered to have the greatest potential to cause HDFN. Laboratory reports include clinical advice on the potential of the antibody to cause HDFN.
• Antibodies such as anti-P1, anti-Lea and Leb, which do not cause HDFN, do not require measurement.

Quantification of anti-D and anti-c

Maternal anti-D and anti-c levels are quantified in international units (IU/mL) by comparison with a national standard. The measurement of antibody in international units is one of the tools available for assessing the likelihood of HDFN. An increase by 50% or greater over the previous level indicates a significant rate of increase, irrespective of the period of gestation.

Generally the significance of anti-D and anti-c levels during pregnancy is as follows:

• Anti-D less than 4 IU/mL HDFN unlikely
• Anti-D 4-15 IU/mL Moderate risk of HDFN, requiring referral to a specialist fetal medicine unit
• Anti-D greater than 15 IU/mL High risk of HDFN, requirement as above

NB: it should be noted that HDFN has been reported at levels less than 4 IU/mL

• Anti-c less than 7.5 IU/mL HDFN unlikely, continue to monitor.
• Anti-c 7.5 – 20 IU/mL Moderate risk of HDFN, requiring referral to a specialist fetal medicine unit
• Anti-c greater than 20 IU/mL High risk of HDFN, requirement as above

The antenatal reports provide clinical advice regarding risk of HDFN and follow-up required. In general, women with clinically significant antibodies should be referred to a hospital obstetric unit. Referral to a specialist fetal medicine unit is advised for women with levels of anti-D or anti-c over 1.0 IU/ml.

The rationale is that significant HDFN can occur with a lower antibody level in a pregnancy if a previous pregnancy has been affected. Referral to fetal medicine is advised for anti-K and other Kell blood group antibodies regardless of level due to the mechanism by which the antibodies cause HDFN. Fetal medicine referral is also advised for any other antibody with a titre of 16 or greater.

Samples from biological father

Determining the relevant blood group of the father of the baby provides useful information when the pregnant woman has anti-D, anti-c or anti-K, or where another antibody has unexpectedly caused severe HDFN in a previous pregnancy. This information can help predict the blood type of the fetus and is useful when giving advice about risk to future pregnancies. For other antibodies, the laboratory will request a sample from the father of the baby when it is necessary.

Fetal genotyping for the management of potential HDFN

It is possible to predict the blood group of the fetus using molecular genotyping by testing a sample of maternal peripheral blood. The likely expression of the following antigens can be predicted using maternal peripheral blood in high-risk cases: RhD, RhC, Rhc, RhE and K (Kell). These tests are undertaken by the NHSBT International Blood Group Reference Laboratory (IBGRL), Filton and full details can be found on the website: International Blood Group Reference Laboratory – NHS Blood and Transplant.

Anti-D prophylaxis

Guidance for the administration of anti-D immunoglobulin has been published as Guidelines for the use of anti-D immunoglobulin for the prevention of haemolytic disease of the fetus and newborn (2014) https://b-s-h.org.uk/guidelines/guidelines/use-of-anti-d-immunoglobinfor-the-prevention-of-haemolytic-disease-of-the-fetus-and-newborn/.

Anti-D immunoglobulin injected intramuscularly begins to enter the blood stream very rapidly (within 1-2 minutes). Unfortunately, laboratory tests cannot distinguish this from immune anti-D formed by someone as a result of being exposed to D positive red cells. Misinterpretation of anti-D results may mean that the mother is deprived of antenatal and postnatal prophylaxis or may result in the failure to monitor an infant for HDFN.

It is therefore essential that the dose of anti-D given, and the date of administration are recorded clearly in the woman’s medical records and that this information is provided on the laboratory request form to avoid misinterpretation of laboratory results. Details of anti-D injected after delivery if samples are referred post-delivery should also be recorded.

It is important that the routine third trimester antenatal testing sample is taken PRIOR to the administration of the 1500 IU dose of routine antenatal anti-D immunoglobulin (RAADP) at 28 weeks. (It is not necessary to wait for the results of the tests before giving the anti-D injection). If any anti-D detected does not meet with specific criteria, the sample is referred to NHSBT for full anti-D quantification (in IU/ml).

When should anti-D immunoglobulin be given?

Routine Antenatal Prophylaxis “Guidance on the use of routine antenatal anti-D prophylaxis (RAADP) for RhD-negative women” issued by NICE in May 2002 and updated in 2008 recommends that routine antenatal anti-D prophylaxis of at least 500 IU at 28 and 34 weeks or 1500 IU at 28 weeks, is offered to all RhD negative women who have not made anti-D. The guidance may be accessed on the NICE website: Overview | Routine antenatal anti-D prophylaxis for women who are rhesus D negative | Guidance | NICE. In this region, the practice is to give one1500IU dose of anti-D prophylaxis at 28 weeks.

Antenatal potential sensitising events (PSE)

In addition to the RAADP, it is recommended that all D negative women should be given prophylactic anti-D following a PSE. This may include childbirth (unless the child is Rh D negative), miscarriage, termination of pregnancy, amniocentesis, abdominal trauma, chorionic villus sampling, ectopic pregnancy, external cephalic version or any event which may have allowed the passage of fetal cells into the maternal circulation. If fetal RHD screening has predicts the fetus is RhD negative, anti-D will not be offered for antenatal PSE’s.

NB. It is not necessary to give prophylactic anti-D during the first 12 weeks of pregnancy in the event of threatened or complete miscarriage which did not require clinical intervention. The normal recommended dose is 250 IU before 20 weeks gestation and 500 IU after 20 weeks gestation.

NB. When anti-D is given after 20 weeks gestation a blood sample must be taken for a Kleihauer test to establish if additional anti-D is required (500 IU is sufficient to cover only a fetal bleed of up to 4mL).

Post-delivery anti-D

The greatest risk of fetal red cells entering the mother’s circulation is at the time of delivery. It is important, therefore, that all D negative women have a cord sample taken to test the infant’s blood group. Those women who bear a D positive child and are not known to have immune anti-D must be given at least 1500 IU of anti-D immunoglobulin as soon as possible but always within 72 hours of the birth.

Prophylactic anti-D (or passive anti-D) may be detected for up to twelve weeks (depending on dose) after administration and should not be a contraindication to giving the postnatal dose if the infant is D positive. A blood sample should be taken from D negative mothers no earlier than 45 minutes after delivery for a Kleihauer test.