The Kleihauer-Betke test was developed to quantify fetal RBCs in maternal blood in order to determine the amount of Rh immune globulin that needs to be administered.
The test is performed by treating maternal red cells on a thin slide smear with acid and then counterstaining the slide. Fetal hemoglobin is resistant to acid treatment so maternal cells will appear as "ghosts " ¯ while fetal cells will be pink.
After counting 2,000 red cells, the results are reported as a percentage of fetal red cells present in the maternal circulation. This result can be multiplied by the maternal blood volume in order to determine the volume of fetal blood (mL) present in the maternal circulation.
Normal range (adult RBCs): <1% fetal Hb.
Interpretation
The presence of fetal RBCs in maternal blood indicates fetal " “maternal hemorrhage.
Limitations
False-Positive Results
Fetal Hb " “containing RBCs may be found in approximately 50% of pregnant women (but in only 1% of pregnancies is the infant anemic).
Certain hematologic disorders in adults, such as leukemias or myelodysplastic syndromes, may increase the level of fetal-type hemoglobin.
Lymphocytes may take up stain in varying degrees.
False-Negative Results
A major blood group incompatibility between mother and infant can cause elimination of fetal RBCs due to hemolysis.