Blocked D phenomenon implicated in a diagnostic dilemma in RhD-hemolytic disease affecting twins: case report and review of literature.

Background: The Rh blood group system is the most common cause of hemolytic disease of the fetus and newborn (HDFN). Rh antigens are fully expressed at birth unlike ABO antigens which are weakly expressed. Sensitization to the D antigen can occur with exposure to < 0.1 mL of fetal blood. In rare cases of HDFN, these passively transferred IgG anti-D antibodies coat the D antigens on the newborn's red blood cells and interfere with the agglutination of D-positive red cells when tested with IgM anti-D typing reagents, resulting in false-negative Rh(D) typing. This "blocked D phenomenon," can pose a diagnostic challenge.

Case presentation: This case report describes twins with HDFN born to a Rh(D) negative mother. Both cord blood and neonatal blood were incorrectly typed as Rh(D) negative using routine typing reagents, creating a diagnostic dilemma. The combination of a positive direct antiglobulin test (DAT), the mother's RhD-negative status, a positive indirect antiglobulin test (IAT), and discordant or unexpected RhD typing in the neonate raised suspicion of blocked D phenomenon. Paired samples from the parents and neonates were analysed. Following gentle heat elution at 45°C for 10 minutes, the neonatal red cells were re-typed as RhD positive using the conventional tube technique with monoclonal IgM anti-D. At the 6-month follow-up, both infants were phenotyped as O RhD positive.

Conclusions: The possibility of the blocking phenomenon should be considered while interpreting blood group results from fetal or neonatal samples in an alloimmunized pregnancy with potent antibodies. All pregnant women, regardless of their RhD type, should be tested for clinically significant unexpected serum antibodies during pregnancy. Elution methods help in identifying correct D antigen when Rh(D) typing gives uncertain results. Antiglobulin testing with anti-IgG should be performed to detect antibodies causing hemolytic disease of the fetus and newborn (HDFN).