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What does it means to have anti D antibody titer of 1:32 in pregnant lady (2child) and what to do next? What’s the normal titer?
Posted Mar 24, 2021, 6:48 PM UTC
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What does it means to have anti D antibody titer of 1:32 in pregnant lady (2child) and what to do next? What’s the normal titer?
Posted Mar 24, 2021, 6:48 PM UTC
Indirect Coombs testing means you take the patient's plasma, then mix it with the lab's sample of RBCs with D antigens on them, and then you add a Coombs reagent.
If there is agglutination, it means that there was antibody binding to the RBCs. In other words, if there is agglutination, it mean's the patient's serum must have had anti-D antibody that bound to the D antigen on the lab's sample RBCs.
How much anti-D antibody is in the patient's serum? Suppose you take 50mL of the patient's serum, and them mix it w 50mL of something else. So now the anti-D antibody concentration is 1/2 of what it was originally. If the indirect Coombs test still yields agglutination at this concentration (1/2), and then you dilute again so that the concentration is 1/4, but it no longer yields a positive result at 1/4 concentration, then you can say the antibody concentration was high enough to yield a positive result at 1/2 concentration, or a titer of 1:2.
However, if it keeps yielding positive results as you dilute it more and more, then the titer might be 1:4, then 1:8, 1:16, 1:32, etc.
If it was positive at 1:32 dilution, that means that the patient's serum had a positive anti-D result at 1/32 of the serum's original concentration but not at 1/64.
If you have absolutely 0 anti-D antibodies, then you titer should be 0. If you have titers of 1:1 or 1:2 (i.e., positive indirect Coombs at the original concentration or 1/2 concentration), then that might not be clinically significant and might not count as being "alloimmunzed". Different labs/societies have different standards for what is considered "alloimmunized" or "sensitized" - for example, some people might say 1:4 or 1:8 (relatively low but non-zero titers). Beyond that, there is a titer level that is considered a "critical value" - this might be 1:16 - 1:32.
So a titer level of 1:32 means the concentration of anti-D antibodies was so high that they had to dilute the serum 5 times before the positive result disappeared - and this level corresponds to some significant risk to the fetus.
What to do next? Well, for one it means that you don't have to give Rhogam anymore because it's already too late. Other than that, there are number of steps to take regarding fetal surveillance, but in the end there aren't simple, widely adopted interventions done to protect the baby from hemolytic disease of the newborn / fetal hydrops, and the specific surveillance guidelines I'm pretty sure are beyond the scope of Step 1 or Step 2