Blocked D Phenomenon

Why would an Rh positive baby type as Rh negative?

Intrauterine transfusion? Sure that's possible. 

This specific scenario describes more of an immunologic basis for mistyping.

Enter...the Blocked D phenomenon. It is something very rarely seen in Blood Banks, especially these days with ever increasing sensitivity of reagents, but it can still happen. 

The Blocked D phenomenon may occur when a mother has created a strong IgG based allo-Anti-D and bears an Rh positive child. It is generally seen in fairly severe cases of HDFN. 

A sample experiencing the blocked D phenomenon will type as Rh negative, and essentially always have a positive IgG DAT. A subsequent elution would show the maternal Anti-D coating the Red Cells of the baby. The red cells type as Rh negative because maternal Anti-D is blanketing the RhD antigens on the Red Cells, blocking the REAGENT IgM Anti-D from agglutinating the cells. As a result, a false negative Rh testing occurs. With a positive IgG DAT, it would not be possible to perform a Weak D (Du) without treating the cells either, as Weak D testing is performed at the AHG (Coombs) phase. 

Cord Blood Evaluations generally arrive to the blood bank as a battery of Blood Type and Direct Coombs testing. This is why it is important to finish all testing prior to resulting. If the blood type was resulted before the DAT was completed, you may be inadvertently reporting erroneous results to the patient's Blood Bank file and chart. 

How to remedy this?

Generally you would need to perform an elution, such as an acid glycine elution, to remove the bound IgG from the neonates Red Cells. Once removed, and you can prove the antibody is removed by perform another DAT to make sure the IgG is now negative, you can retest the red cells for the D antigen using your monoclonal typing reagents. You may now perform a Weak D test as well, since the DAT is no longer IgG positive. 

How many of you have seen this phenomenon? While rare, it has real implications in the Blood Bank and for proper patient care. Not realizing the phenomenon taking place, can result in a delay of care. A newborn exhibiting this phenomenon may need to receive an exchange transfusion to remove their Red Cells and replenish with Rh negative cells until the maternal antibody is no longer reacting. If the phenomenon is not noticed at first, and the baby is resulted as Rh negative, it will change the clinical picture for the physician and make it more difficult for them to arrive at a diagnosis of HDFN and treat at a earlier point in time.