The postpartum dose of Rh immune globulin varies according to an individual laboratory estimation of fetal red blood cells in each mother’s peripheral blood. In the United States, a four-step procedure determines the postpartum dose (number of vials of 300 micrograms; 1,500 international units) of Rh immune globulin (anti-D) for each RhD-negative mother who has delivered an RhD-positive newborn and has not already formed anti-D. The first step is a rosette fetal red blood cell screen to determine whether an excessive (greater than 30 mL fetal whole blood) fetomaternal hemorrhage occurred. If the rosette screen is negative, the mother receives one vial of Rh immune globulin for Rh immunoprophylaxis. If the rosette screen is positive, the blood sample is retested by a quantitative method, typically an acid-elution (Kleihauer-Betke) assay. The result of the acid-elution assay is converted to an estimation of the volume of the fetomaternal hemorrhage, which is the basis for calculating the dose of Rh immune globulin. The acid-elution assay is subjective, imprecise, and poorly reproducible. As a result, the formula for calculating the dose includes a precautionary adjustment, adding an extra vial in borderline situations to prevent underdosing. Flow cytometry is a more precise method for quantifying a fetomaternal hemorrhage. However, few hospitals use flow cytometry, because it is not cost-effective to maintain an expensive, high-technology laboratory service for the relatively few occasions when a precise quantitative determination of fetomaternal hemorrhage is required.