This test quantifies fetal red blood cells in maternal circulation. It is based on the principle of acid elution, where adult hemoglobin is removed from red blood cells on a prepared blood smear, while fetal hemoglobin remains. Microscopically, the fetal cells, stained pink, are counted against a backdrop of faintly visible adult “ghost cells.” A standardized formula, incorporating the total number of fetal cells and the number of maternal cells counted, determines the percentage of fetal blood volume in the maternal sample. For example, if 5,000 maternal cells are counted and 100 fetal cells are identified, the calculation would be (100/5000) * 100 = 2% fetal cells.
Determining the volume of fetomaternal hemorrhage is crucial for appropriate management, especially in cases of Rh incompatibility. The test enables accurate dosing of Rho(D) immune globulin (RhIg), preventing potential alloimmunization in Rh-negative mothers carrying Rh-positive fetuses. Developed by Enno Kleihauer, Herbert Betke, and Erika Kleihauer in 1957, this acid elution technique revolutionized the prevention of hemolytic disease of the newborn. Historically, a standard dose of RhIg was administered; however, this method allows for tailored dosing based on the extent of the bleed, maximizing protection and minimizing unnecessary exposure to blood products.