What is the calculation for determining the need for additional RhIg based on fetal blood volume?

To determine the need for additional Rh immunoglobulin (RhIg) based on fetal blood volume, the calculation involves the use of the Kleihauer-Betke (KB) stain, which estimates the percentage of fetal red blood cells in maternal circulation. This is crucial for assessing the extent of fetomaternal hemorrhage (FMH) that might require supplemental RhIg to prevent Rh sensitization in Rh-negative mothers.

The correct calculation uses the formula: KB Stain % x 50 = volume of fetomaternal hemorrhage in milliliters. This means that for every 1% of fetal red blood cells found in the maternal circulation, approximately 50 mL of fetal blood has entered the maternal system. Therefore, if a KB stain reveals, for example, 2% fetal cells, then the calculated volume of fetomaternal hemorrhage would be 2 x 50 = 100 mL.

This formula is rooted in clinical practice, reflecting the average volume of fetal blood within maternal circulation and is particularly important in managing pregnancies at risk for hemolytic disease of the newborn. It serves as a key tool in determining the appropriate amount of RhIg to administer in order to effectively protect the mother and future pregnancies.