Kleihauer-Betke test

The Kleihauer–Betke ("KB") test, Kleihauer–Betke ("KB") stain, 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. It 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. It's named after Enno Kleihauer and Klaus Betke who described it in 1957.

The KB test is the standard method of quantitating fetal-maternal hemorrhage (FMH). It takes advantage of the differential resistance of fetal hemoglobin to acid. A standard blood smear is prepared from the mother's blood, and exposed to an acid bath. This removes adult hemoglobin, but not fetal hemoglobin, from the red blood cells. Subsequent staining, using Shepard's method, makes fetal cells (containing fetal hemoglobin) appear rose-pink in color, while adult red blood cells are only seen as "ghosts". 2000 cells are counted under the microscope and a percentage of fetal to maternal cells is calculated.

In those with positive tests, follow up testing at a postpartum check should be done to rule out the possibility of a false positive. This could be caused by a process in the mother which causes persistent elevation of fetal hemoglobin, e.g. sickle cell trait.

Comparison with other more expensive or technologically advanced methods such as flow cytometry has shown that the KB stain, like the more advanced methods, is sensitive in its detection of FMH. Background counting errors can result in estimates of as much as 5 ml fetal blood loss when there actually is no such blood loss, but standard methods available in most laboratories admit an extremely low probability of the return of a false positive when more severe FMH has taken place.

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