Rhesus Alloimmunization

Question 1

what constitutes a maternal allo-antibody

Answer 1

an antibody against antigen the mother does posssess herself (ie. against the fetus)

Question 2

what are the potentially harmful RBC antigens?

Answer 2

Rhesus C/D/E (85%), Kell, Duffy, Kidd, MNS

Question 3

epidemiology of alloimmunization in pregnancy?

Answer 3

1% will experience, 85% of these will related to Rhesus antibody.

15% of women are Rh negative.

Question 4

how can severity of Rbc alloimmunization be predicted?

Answer 4

based on Ab titre:
mild <32
moderate 64-256
severe >512

also corresponds to increased perinatal mortality and fetal hydrops

Question 5

how should rbc iso-immunization be assessed?

Answer 5

Dx

1. first antenatal visit:
   - screening for Rh antibodies
   - risk allocation–>Ab titre
   - *partner grouping–>if partner does not have relevant antigen (ie. dd group, not DD or Dd) can reassure the couple and return to normal risk pregnancy

• if partner is heterozygous, perform fetal dna typing:
• via amnioscentesis (avoid CVS, can cause fetomaternal hemorrhage)
• cell free fetal dna sampling from maternal blood

2.Rh negative women should be screened again at 28 weeks and delivery

Question 6

how can rbc isoimmunization be managed (Mild/mod/severe)?

Answer 6

Mgx

• prophylactic anti-d at 28 and 34 wks gestation (90% effective)
• deliver at 38 weeks to reduce risk of feto-maternal Ab transfer (or earlier if fetal anaemia)

Mild risk (Ab <32):
-check Ab titre at each visit (will rise quickly in instance of feto-maternal hemorrhage

Moderate (Ab 64-256):

• as above and,
• U/S from 20wks for MCA peak systolic velocity (anaemia will cause reduced blood viscosity –>vasodilation and increased blood velocity)
• CTG from 32 weeks (MCA PSV less reliable at this stage)

Severe (Ab >512)

• as above and,
• U/S screening from 17 weeks
• fetal blood sampling (if PCA MSV raised)
• intrauterine transfusion if fetal anaemia

Question 7

How can RBC isoimmunization be prevented?

Answer 7

• ensure Rh compatible blood given to women who require transfusion
• ensure Kell negative blood given in transfusion
• administer passive anti-D to Rh(D) negative women at times of sensitive events (ie abortion; bleeding in pregnancy; trauma in pregnancy: amnioscentesis, CVS, MVA; post-delivery)

Question 8

what is the Kleihauer test used for?

Answer 8

is a blood test used to measure the amount of fetal hemoglobin transferred from a fetus to a mother’s bloodstream. (feto-maternal hemorrhage). Can titrate anti-D accordingly.