The Kleihauer-Betke test is used to estimate the volume of a fetal-maternal hemorrhage. Fetal RBCs are rich in fetal hemoglobin, and fetal hemoglobin is resistant to acid elution. Thus staining for hemoglobin after an acid elution will stain RBCs with fetal hemoglobin, while RBCs with normal hemoglobins will be very faint.


After staining, the number of deeply fetal red blood cells is counted relative to a number of faintly staining maternal red blood cells (usually 2000 cells), and a percentage calculated. relative the total number of RBCs counted (maternal + fetal).


milliliters of fetal blood in maternal circulation =

= (percentage of fetal red cells) * 50



• The percentage of red cells is a whole number, not the fraction (example: 2% fetal RBCs uses 2).


Falsely elevated estimates of the hemorrhage may occur:

(1) in mothers with elevated fetal hemoglobin;

(2) undercounting the unstained maternal RBCs (the denominator)


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