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rh_isoimm

Rh isoimmunisation and Anti-D

Introduction

  • all patients who are Rh -ve and are pregnant are at risk of developing Rh iso-immunisation which is the development of permanent maternal antibodies against fetal Rh +ve red blood cell markers. Once this occurs fetal haemolytic anaemia will result (hemolytic disease of the newborn or HDN) causing erythroblastosis fetalis which tends to be worse in magnitude with successive pregnancies and may cause fetal demise.
  • although the rate of iso-immunisation developing in an otherwise uneventful pregnancy is approximately 1-2%, mismanagement of patients who present with increased risk may result in significant medicolegal issues.
  • risk factors for iso-immunisation sensitization and the potential indications for Anti-D are any circumstance that may allow fetal red blood cells into maternal circulation such as:
    • confirmed miscarriage requiring D&C (NB. Anti-D is no longer needed for threatened miscarriage in 1st 13 wks if bleeding is mild with minimal pain)
    • uterine bleeding which has been heavy, repeated and/or associated with pain
    • molar pregnancy
    • medical procedures such as amniocentesis, chorionic villous sampling, termination of pregnancy, fetal rotation
    • abruption
    • placenta praevia
    • trauma to pregnant abdomen
    • Caesarian section or vaginal delivery (send cord blood as well)

Historical background

  • 17% of Caucasians are Rh neg and thus 5 in 6 Rh negative Caucasian women will have a Rh positive partner, 45% of which will be homozygous and 55% will be heterozygous - if homozygous, the fetus will be Rh pos, if heterozygous, there is a 50% chance the fetus will be Rh pos.
  • prior to Anti-D availability, an Rh neg women would have a 16% chance of becoming alloimmunized for Rh D antigen after the birth of 2 Rh pos infants (the use of routine post-partum Anti-D dropped this to 2% and the use of routine Anti-D in the 3rd TM further dropped this to 0.2%)
  • prior to Anti-D availability, couples often had Rh testing before they were married to ensure compatibility
  • the use of Anti-D has dropped HDN mortality due to Rh incompatibility from 46/100,000 births to 1.6/100,000 births
  • Anti-D was then indicated for any potentially sensitizing event including any PV bleeding in pregnancy - the indications have recently been relaxed now to generally exclude threatened miscarriages in 1st TM and the dose has been reduced for the 1st TM events.

Prevention:

  • routine 625IU Anti-D administration at:
    • 28wks
    • 34wks
    • post-partum
  • sensitizing event administration of an appropriate dose of Anti-D within 72hrs of the risk event commencing (can be given up to 10 days but less effective)
  • if intramuscular injection (IM) is contraindicated (severe thrombocytopenia, therapeutic anticoagulation, hereditary bleeding disorder), Rhophlyac 1500 IU should be administered intravenously.
  • NB. mini-dose Anti-D of 250IU commenced in Australia in May 2001 due to shortages of Anti-D, this is sufficient to prevent immunisation by a fetomaternal haemorrhage of 2.5ml of RBC's (ie. 5ml of whole blood) and will cover any event in 1st trimester.

Management of sensitising events in pregnancy:

  • in any Rh -ve patient who has the above risk factors, and are not known to be already iso-immunised (ie. do not already have Rh antibodies), she should be managed as follows:

1st trimester:

  • if no Rh Anti-D has been given within 1-2 weeks (the usual duration of action):
    • check Rh antibody levels:
      • if no antibodies give mini-dose Anti-D 250IU (625IU if multiple pregnancy)
      • if antibodies present:
        • confirm with laboratory that they were from recent Anti-D dose and whether sufficient amount:
          • if insufficient levels from recent Anti-D, consider repeat mini-dose AntiD
        • if maternal antibodies then refer to O&G for follow up and advice

2nd or 3rd trimester:

  • check Rh antibody levels and request Kleihauer test (FBE tube, measures fetal RBC and assists with Anti-D dosing):
    • if no antibodies:
      • give Anti-D 625IU or more if indicated by Kleihauer test (this test is only needed if 20wks gestation or more)1)
      • NB. patients with BMI > 30 and a feto-maternal haemorrhage > 6mL may require additional iv dose to increase bioavailability and facilitate more rapid clearance of fetal cells
      • Women with continued PV bleeding between 12 and 20 weeks gestation, should be offered Rh-D Ig at a minimum of 6-weekly intervals
    • if antibodies present:
      • confirm with laboratory that they were from recent Anti-D dose and whether sufficient amount:
        • if insufficient levels from recent Anti-D, consider repeat Anti-D 625IU
        • if maternal antibodies then refer to O&G and haematologist ASAP for follow up and advice
1)
Qureshi, et al., 2014
rh_isoimm.txt · Last modified: 2019/09/04 05:11 by wh