The Kleihauer-Betke (KB) test is a laboratory blood test primarily used during pregnancy to detect and quantify the amount of fetal blood (fetal red blood cells) that has entered the maternal circulation. This is especially important in cases of suspected fetomaternal hemorrhage (FMH), which can occur due to trauma, delivery, or certain obstetric events.
How the Test Works
The test exploits the fact that fetal red blood cells contain fetal hemoglobin (HbF), which is more resistant to acid than adult hemoglobin (HbA).
A blood smear from the mother is treated with acid. Adult red blood cells lose their hemoglobin and appear as pale "ghost" cells, while fetal cells retain their hemoglobin and stain bright pink or red.
A technician counts the number of fetal cells versus maternal cells under a microscope, typically reviewing about 2,000 cells.
The percentage of fetal cells is calculated and used to estimate the volume of fetal blood in the maternal circulation.
Clinical Uses
Determining Rh Immune Globulin (RhIG) Dosage: Most commonly, the KB test is used in Rh-negative mothers to calculate the appropriate dose of RhIG needed to prevent Rh sensitization and hemolytic disease in future pregnancies.
Assessing Fetomaternal Hemorrhage: It helps quantify FMH after trauma, delivery, or other obstetric incidents, guiding management and prognosis.
Other Uses: It can also help explain fetal anemia and is sometimes used in the evaluation of certain hemoglobinopathies, though it is not a screening tool for fetal anemia.
Limitations
The KB test is considered somewhat crude and can be challenging to interpret, with results varying between observers.
It may overestimate FMH in cases where the mother has elevated levels of fetal hemoglobin due to conditions like sickle cell trait or other hemoglobinopathies.
It has lower sensitivity and reproducibility compared to more advanced methods such as flow cytometry, but remains widely used due to its simplicity and availability.
Summary Table
Key Points
The Kleihauer-Betke test is a vital tool in obstetric care for managing Rh incompatibility and assessing fetomaternal hemorrhage.
It is based on the differential acid resistance of fetal versus adult hemoglobin in red blood cells.
While widely used, it has notable limitations in accuracy and reproducibility compared to newer techniques.
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