The KB test is performed to quantitate the number of fetal cells present in the maternal circulation. Once the size of the feto-maternal hemorrhage (FMH) is determined, the appropriate RhIG dose can be calculated and administered to prevent the mother from making anti-D.
The test is based on the principle that red cells containing fetal hemoglobin (HbF) are less susceptible to acid elution than cells containing adult hemoglobin (HbA).
- A peripheral blood smear is made from the maternal postpartum sample and treated with acid. Fetal cells remain intact because of high concentrations of HbF, while HbA is eluted from the maternal cells.
- After acid treatment the slides are washed, stained, and examined microscopically.
- The number of fetal cells (which take up the stain) are counted per number of maternal cells (which appear as ghost cells) to give % fetal cells.
- The volume of fetal bleed is then calculated to determine how much additional RhIG is required.
The top image on the right illustrates a negative KB test. The blue arrows in the bottom image point to fetal cells that have taken up the stain. The red arrows indicate maternal cells, which appear as ghost cells.
These images were provided courtesy of Mount Sinai Blood Transfusion Laboratory, Toronto, Ontario.
Limitations: Despite its widespread use, the KB test has significant limitations, including
- Low sensitivity
- Poor reproducibility.