Which formula is used to determine the volume of fetomaternal hemorrhage expressed in mL of whole blood based on Kleihauer-Betke test results?

The formula used to determine the volume of fetomaternal hemorrhage expressed in milliliters of whole blood based on the Kleihauer-Betke test results is indeed based on the percentage of fetal cells present in the maternal circulation.

In this context, the Kleihauer-Betke test is a quantitative measure that identifies the presence of fetal red blood cells (RBCs) in maternal blood. The percentage of fetal cells is directly correlated to the volume of fetal blood that has entered the maternal circulation. To convert this percentage into an actual volume, a specific multiplier is applied. The correct multiplier in this case is 50, which is derived from the formula:

% of fetal cells present x 50 = Volume of fetal hemorrhage in mL.

This means for every 1% of fetal cells detected, there are approximately 50 mL of fetomaternal hemorrhage. The choice of 30 or a reflection of maternal cells’ percentage does not accurately represent the physiological relationship necessary for calculating the volume of fetal blood in maternal circulation. Therefore, using the percentage of fetal cells with a multiplier of 50 gives correct measurement in clinical practice for assessing the extent of hemorrhage and potential impacts on both the mother and fetus