Elimination of ABO Blood Group Incompatible Fetal Red Blood Cells in the Maternal Circulation: Relevance to the Diagnosis of Fetal-Maternal Hemorrhage.

Background: Fetal-maternal hemorrhage (FMH) occurs when fetal red blood cells (RBC) pass into the maternal circulation as a result of obstetric- or trauma-related complications to pregnancy. Their detection in the maternal blood is commonly used as a diagnostic test. There is, however, a serious and general limitation to this test that is sometimes ignored. Fetal RBC carrying the father's antigens (most crucially, the ABO blood antigens) may be incompatible with the mother's plasma. They are expected to be eliminated by the maternal natural antibodies, thus, negative results may be false.

Objectives: By simulating fetal-maternal ABO incompatibility, we studied the fate of fetal RBC in vitro.

Methods: Adult blood samples (n = 6) of O-blood group (type) were mixed with 1-5% cord blood or neonatal blood of A- or O-type, representing incompatible and compatible fetal RBC, respectively. The survival of fetal RBC was quantified after an overnight incubation. The supernatant was assayed for fetal hemoglobin (HbF) using the spectrophotometric alkaline-resistance benzidine assay, while the pellet was assayed for HbF/carbonic anhydrase (CA) expression in RBC by flow cytometry. The HbFhigh/CAlow phenotype characterizes fetal RBC.

Results: Both assays demonstrated disappearance of the fetal RBC due to lysis upon incubation in incompatible blood.

Conclusions: A similar situation may also occur in vivo. Thus, under these conditions, negative results in the FMH test may be false, and lead to misdiagnosis.