We have had several patients recently who are DAT positive with a C3 coating. When the ID panel is set up by IAT using IgG cassettes they give 2/3+ reactions with all panel cells but the patient's auto control is negative. These patient all have a cold auto agglutinin. I can understand that the cold auto antibody could  still be detected in IgG cards at 37°C but I can't understand why if its an auto antibody the plasma wouldn't react with their own cells. We have only recently changed to using IgG cards and so probably would not have seen it in the past as the A/C3 in the AHG would have caused a positive reaction with the patient's cells. Can anyone explain this? What concerns me is that these patients could also have an antibody to a high incidence antigen although the reactions though strong also have alot of fibrin at the top of the wells.

Well, I can certainly explain the first bit.  Cold-reacting antibodies sensitise red cells very quickly indeed.  Even if you pre-warm your cassettes, the chances are that the air at the top of the cassette (the reaction chamber) is going to be well below 37^oC.  It is probable, therefore, that the red cells are sensitised before the cassettes are incubated at 37^oC.  Once sensitised, the antibody takes quite some time to come back off the red cells - longer than the incubation time.  The cassette is then centrifuged at room temperature, and so you detect the cold-reacting antibody, even in cassettes with monospecific anti-IgG.  It is probable that the antibody is actually an IgM, although cold-reacting antibodies can be IgG (classically, the anti-P in PCH is a biphasic cold-reacting IgG antibody).

 

Turning to the other bit (why the auto is negative), it could just be that the antigens are blocked by so much antibody being on them that the antibody cannot cross from one red cell to the other to form agglutination.  If you like, it is similar to the prozone effect.

I think maybe because the panel cells is group O , and if those patient who produce anti-HI or anti -H cold auto, they maybe group A or AB, the auto react stronger with O than the auto-cells.

Excellent point shily.

Thanks for your replies. I will check what ABO groups the patients were.

Well, I can certainly explain the first bit.  Cold-reacting antibodies sensitise red cells very quickly indeed.  Even if you pre-warm your cassettes, the chances are that the air at the top of the cassette (the reaction chamber) is going to be well below 37^oC.  It is probable, therefore, that the red cells are sensitised before the cassettes are incubated at 37^oC.  Once sensitised, the antibody takes quite some time to come back off the red cells - longer than the incubation time.  The cassette is then centrifuged at room temperature, and so you detect the cold-reacting antibody, even in cassettes with monospecific anti-IgG.  It is probable that the antibody is actually an IgM, although cold-reacting antibodies can be IgG (classically, the anti-P in PCH is a biphasic cold-reacting IgG antibody).

 

Turning to the other bit (why the auto is negative), it could just be that the antigens are blocked by so much antibody being on them that the antibody cannot cross from one red cell to the other to form agglutination.  If you like, it is similar to the prozone effect.

Edited June 3, 201510 yr by RollSlow10

Would a wash with 4 degree saline bring up the Auto or is this redundant?

I don't think it is necessary to do 4 degree saline wash in this case, unless you want to do cold adsorption.

It is redundant.