Using the estimated volume of fetal bleed determined by the KB test or flow cytometry, the number of vials of RhIG (300 µg) to inject is calculated as follows:
- Number of vials of 300 µg RhIG = Volume of fetal bleed/30 mL
In the interests of safety, some American organizations recommend the following to deal with decimal points:
- If the number to the right of the decimal point is <5, round down and add 1 vial (e.g., 1.4 = 1 + 1 = 2 vials).
- If the number to the right of the decimal point is ≥5, round up and add 1 vial (e.g., 1.7 = 2 + 1 = 3 vials).
Sub-calculations:
- Volume of fetal bleed: % fetal cells x maternal blood volume
- Maternal blood volume: 70 mL/kg x weight (kg) (assume 5,000 mL if maternal information is unknown)
