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So what’s the deal with HDFN?
This is sort of a intro to the disease and a less slapdash intro to pre-natal testing. I personally am not very excited by the prenatal part of it, but it’s important. I tried to do this assuming very limited knowledge of blood banking, but it is kind of an esoteric discipline so please ask if something I wrote has completely boggled you.
There’s two kinds, one is ABO HDFN, which is very mild and is the result of an ABO incompatibility between the mother and the baby. Most of the time, this isn’t even detected by physicians except in routine neonatal bloodbank testing.
The picture from my last post probably gave you a good idea of the other. IgG antibodies have the distinct ability to cross the placenta, so if the mother has any blood group antibodies of the IgG variety(usually from a transfusion or exposure in a previous pregnancy) and the baby happens to have the corresponding blood group antigen, it will end up destroying the baby’s blood cells (erythroblastosis fetalis). The hemolysis is at its peak at birth but will decline once the maternal antibodies in its circulation go down. The anemia may be severe enough that the baby will develop severe edema in utero (hydrops fetalis), or jaundice after it is born (icterus gravis neonatorum).
There’s… a lot of blood groups out there, which I won’t get into right now, but HDFN is usually a result of Rh antigens and happens in a huge spectrum of severities. Usually it is against the D which is the most immunogenic blood group antigen after ABO (which is also why we include it in a routine blood typing). Sometimes it is anti-c, which is part of the same blood group. Anti-K from the Kell group can also cause severe HDFN but is rare because most people don’t have a K antigen to react.
So that aside! Let’s talk about prenatal testing because no one wants their babies to get sick with this:
The way it happens here is that Canadian Blood Services will take two tubes from mom in her first trimester: one chemistry tube for serology testing so the baby doesn’t contract hepatitis/syphilis/HIV/rubella/varicella. The other, in EDTA, is used to determine her blood type and to do an antibody screen/ID to look for antibodies. This can go several ways:
If the screen is negative and she is D pos, then everyone is happy and we all move on with our lives.
If the screen is negative but she is D negative, then she gets a shot of this lovely product we call WinRho (AKA anti-D or RhIg).
WinRho is adminstered automatically at 28weeks where there is a spike in the risk of a bleed, even if we have no idea what the ABORh of the baby is. The mother might become immunized if the baby bleeds into her, and if it is D positive, any other pregnancies down the line may become very difficult for HDFN reasons.
The product itself is pooled anti-D from donors (but I think there is a manufactured product available) which will bind the baby’s cells before her lymphocytes can react to them, so she will not make her own antibodies. 300ug of WinRho can clear about 30mL of whole blood.
If she is D neg and is screen positive (anti-D or not), then this is considered a critical and her pregnancy is monitored by an obstetrics specialist the whole way through. She does NOT get WinRho if she is screen positive for anti-D because she is actively producing her own and doesn’t need more!
They will do a titration of her antibody to see what the level is. In short, doubling dilutions of the serum will be made and the inverse of the highest dilution that the antibody reacts is her titre:
This level actually does not matter because it is an arbitrary value anyways. A new blood draw will be made every month and titrated along side a re-test of the original. If the titre increases 4x from the original, this is considered a significant increase and suggests 1) the baby is bleeding into the mother and/or 2) the mother is actively mounting an immune response to it and the obstetrician will intervene.
And that’s the wonderful world of prenatal blood banking. Next: The much, much more exciting world of post-natal blood bank testing.