Labeling D+ RBCs for flow cytometric quantification of fetomaternal hemorrhage after the RBCs have been coated with anti-D

BACKGROUND: D- patients may receive Rh immunoglobulin (RhIg) before blood samples are taken for estimation of the volume of fetomaternal hemorrhage (FMH) by flow cytometry. Anti-D bound to the fetal D+ cells may then block the binding of conjugated D MoAb. This may reduce the fluorescence of the D+ cells, which would lead to ambiguity over setting the positions of the markers on histograms and may result in erroneous values of FMH. STUDY DESIGN AND METHODS: Labeling methods were compared by using FITC-BRAD 3 (anti-D) and/or FITC-anti-IgG (Fab fragment) with mixtures of D+ (R1r) and D- (rr) cells when the D+ cells had first been coated with various amounts (0 molecules/cell and 600-13,000 molecules/cell) of anti-D (RhIg). Variables examined were antibody concentrations, the order and times of incubation with the antibodies, and the effect of washing between the uptake of the antibodies used. RESULTS: In all cases, D+ cells were strongly labeled after incubation with 50 microL of FITC-BRAD-3 and then after washing with 50 microL of FITC-anti-human IgG, with both incubations being for 30 minutes at 37 degrees C. With this double-staining procedure, the fluorescence of D+ cells was found to be similar regardless of how much anti-D (RhIg) was previously bound and greater than that with FITC-BRAD-3 alone, giving an enhanced signal-to-noise ratio. CONCLUSION: As the testing laboratory may not know if the patient has received prophylactic RhIg, this labeling method would be suitable for all samples.