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Hey, I was wondering if you could help with a concept re: Coombs test

In FA 2016 p. 392, it says that the Direct Coombs test will be negative for PNH
I'm curious why this is the case, since isn't PNH complement mediated, and doesn't the direct Coombs test look for complement bound to RBC?


Any info would be appreciated. Thanks!
 

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Hi there,

This is actually a great question! :)

In PNH, erythrocytes are CD55/CD59 negative. As a result of that, complement-mediated intravascular hemolysis ensues. Extravascular hemolysis (namely, C3-opsonized erythrocytes that circulate and can be identified as such, before they are picked up by the spleen for phagocytosis) contributes a small, inconspicuous part. That being said, when PNH patients are treated with eculizumab (anti-CD55 mAb) and erythrocytes are protected from intravascular hemolysis, >50% of them become DAT positive.
If you're not that interested in the mechanism, you can just remember that DAT is only positive in immune-mediated hemolytic anemia and PNH isn't that.

Hope this helps.
 
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