This laboratory procedure is a quantitative assay used to determine the presence and amount of fetal red blood cells in the maternal circulation. The test relies on the differential acid elution of adult hemoglobin from red blood cells, while fetal hemoglobin remains resistant. After acid elution, the fetal cells are stained and can be counted using microscopy. The calculation then determines the percentage of fetal cells in the maternal blood, providing a measure of the extent of fetomaternal hemorrhage. For instance, identifying 10 fetal red blood cells per 50 microscopic fields informs the proportional existence of fetal blood within the maternal system, allowing for a crucial calculation.
Accurate quantification of fetal-maternal hemorrhage is critical for determining the appropriate dose of Rh immunoglobulin (RhIg) to administer to Rh-negative mothers after delivery or any potentially sensitizing event. This is essential to prevent Rh sensitization, a condition where the mother develops antibodies against the fetal Rh-positive red blood cells, potentially leading to hemolytic disease of the fetus and newborn in subsequent pregnancies. Historically, before the widespread use of this calculation, Rh sensitization was a significant cause of perinatal morbidity and mortality. Early and precise quantification of fetal cells allows for more specific and effective RhIg administration, significantly minimizing the risks of sensitization and improving maternal and fetal outcomes.
