Kleihauer–Betke test

The Kleihauer–Betke test, Kleihauer test or acid elution test is a blood test used to measure the amount of fetal hemoglobin transferred from a fetus to a mother’s bloodstream.

The KB test is usually performed on Rh-negative mothers to determine the required dose of Rho(D) immune globulin (RhIg) to inhibit formation of Rh antibodies in the mother and prevent Rh disease in future Rh-positive children.

KB test, shows  fetal red blood cells in rose-pink color, while adult red blood cells are only seen as ghost cells.

It measures fetal hemoglobin transfer.

The KB test is the standard method of quantitating fetal–maternal hemorrhage (FMH). 

A standard blood smear is prepared from the mother’s blood and exposed to an acid bath, removing adult hemoglobin, but not fetal hemoglobin, from the red blood cells. 

The Shepard’s method of staining makes fetal cells containing fetal hemoglobin appear rose-pink in color, while adult red blood cells are only seen as ghost cells.

2,000 cells are counted under the microscope and a percentage of fetal to maternal cells is calculated.

Smears are prepared from capillary blood or venous blood collected into anticoagulants.

Hemoglobin F cells are densely stained with erythrosin.

Hemoglobin A cells appear as ghost cells, while intermediate cells are stained more or less pink. 

Reticulocytes containing Hemoglobin A may appear as intermediate cells and/or may show intracellular granulation. 

Normal values for Hemoglobin F cells in adults are below 0.01%; in full-term newborns they are above 90%.

The size of a fetal red blood cell is 1.22 times that of an adult red blood cell.

The KB stain is known to have a mean success rate of 92% in detecting fetal red blood cells.

The number of RHD vials required is based on the volume of fetal blood discovered, with the number of vials of 30 µg of Rh IG required equal to the volume of 30 mL of fetal blood.

A Kleihauer-Betke (KB) test is necessary in all cases of maternal trauma, as it accurately predicts the risk of preterm labor after maternal trauma: With a positive KB test, the significant risk of pre-term labor mandates detailed monitoring.

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